There's a game they play at the stock market called Thursday Afternoon Quarterback. Every Thursday, the Federal Reserve Board publishes figures on the latest weekly squiggle in money supply growth. The statistics are virtually meaningless - even the Fed warns they often are misleading. Yet the markets react widly to them, taking them as a signal whether to buy or sell.

Government policymakers used to laugh at his sort of mindless reaction, but last week Washington showed it may be succumbing to the same malady. Congress, which has been on again and off again over whether and by how much to cut taxes, is moving to trim the size of President Carter's $25 billion tax cut plan. And Carter signaled Friday that he is ready to follow.

In a formal statement, the President announced a compromise with House and Senate budget committee leaders that calls for paring Carter's tax cut to $19.4 billion and delaying its effective date to Jan. 1 instead of Oct. 1. The net effect: to slash the tax reduction for fiscal 1979 to $15 billion. The rationale: to assuage inflation fears and avert another interest rate rise.

At issue is not whether the cutback is sensible. Indeed, a good many economists, both liberal and conservative, have argued that the administration should back away from its earlier $25 billion net tax reduction. The figure Carter proposed last January was regarded even then as too hefty. And the economy is proving somewhat more vigorous than had been expected.

The real question is, why did Carter cave in - for genuine economic reasons, or just as another political face-saving gesture, or, in this case, to millify the Federal Reserve? This is the fourth time the president has made a major alteration in his broad economic policy since his election - and it clearly is not repairing the administration's widely-criticized image of inconsistency.

Indeed, when news of Carter's wavering on the tax cut issue emerged a few weeks ago, the first reaction was to recall his handling of the $50 income tax rebate last year. Carter proposed the $50-a-person rebate as part of his economic stimulus package in early 1977, but withdrew it abruptly in May in the face of political opposition. The reversal left many congressmen hanging.

This latest turnabout comes in the face of stiff - and widely predicted - opposition to Carter's 1978 tax package in the House Ways and Means Committee, which also has been backpedaling amid voter concern about inflation. It seems almost inevitable that the panel now will trim Carter's net cut to well below $20 billion and use some of the money to cut payroll taxes.

What makes everyone suspicious is that the switch comes only 1 1/2 weeks after Carter's own economists revised their January forecasts to show that the economy will grow more slowly this year than expected. Although the forecast also shows more inflation is likely, the price surge stems from farm prices and the dollar devaluation, neither of which is exacerbated by a tax cut.

Moreover, as late as April 25, Carter angrily denied earlier reports that his top policymakers were considering a reduction in the tax cut package - although they were - and vowed that he had no intention of backing away from his January proposals. Now, a year after his withdrawal of the $50-a-person tax rebate, it seems that Son of Rebate is here.

Admittedly, the arguments for trimming the tax cut sails are persuasive. For one thing, the economy is performing decidedly better than had been expected, while the unemployment rate has fallen more rapidly. For another, inflation is creeping steadily higher, and the Fed is threatening to raise interest rates further unless the administration acts on its won.

More important, even many liberal economists now believe Carter erred back in January by planning the fiscal 1979 budget deficit to turn out higher than this year's, during a period when the economy is beginning to run into capacity problems in some industries. Walter Heller, a former Kennedy economic adviser, sought a $35 billion tax cut in January but now is calling for a delay.

Otto Eckstein, the onetime Johnson administration economic adviser, notes that the "full-employment" sdeficit - the key measure of how much fiscal stimulus the government is providing - will rise to $26.5 billion in fiscal 1979 from $16.3 billion in fiscal 1978 and $8.7 billion in fiscal 1977 at a time when, by all rights, it ought to be declining.

At the same time, however, the picture still is not quite that decisive. Many economists caution that the outlook for 1979 and later still is uncertain, with the possibility that the recovery could slow to a pace where the extra stimulus might come in handy. And although trimming the tax cut package might help symbolically, it won't have much direct impact on inflation.

The administration's decision to go along with the congressional cutback effort came about suddenly - "almost overnight," according to one sources - on Wednesday afternoon and Thursday. Treasury Secretary W. Michael Blumenthal and budget director James McIntyre had been urging such action for several weeks, but the White house had resisted.

To be fair, this sort of seesaw economic policymaking hasn't been confined to the administration. Congress also has played the Thursday afternoon game on tax policy - first reversing a payroll tax increase in May that it passed the previous December, and, in the latest gambit, toyning with a proposal aimed at scuttling the entire income tax cut in toto.

And, as Charles Schultze, the president's chief economic adviser, told reporters on Friday, the government's economic policysetting machinery isn't fast enough to respond quickly to change in the condition of the economy. What Carter proposed last January was based on November's economic analysis. By the time the tax cut gets through Congress, it could be October.

But Congress - especially the House - is especially vulnerable to the whims of its constituents, who have shifted their worries since December from the Social Security trust fund and unemployment to payroll taxes and inflation. What the lawmakers appear to have done is to link the concern about inflation with the size of the tax cut, ignoring the need to sustain growth.

The White House is supposed to be above that fray - and able to stand up to the will-o'the-wisp pressures of congressmen and constituents. If the White House is right now solely on economic grounds, why was it wrong last january? If it merely is bowing to political pressures, why isn't the administration stronger and better able to judge longer-range trends?

Analysts argue that while the economy may seem to be bouncing back sharply this quarter, it's likely to slow further later this year than the january forecasts had anticipated. Since the administration condedes that paring the tax cut won't help slow consumer prices, the question is, why change basic policy on the basis of a few weeks' rosy statistics?

When Carter announced his new economic proposals last January, he told the nation that, despite his flip-flop image of the year before, he now was ready to settle on a set of comprehensive and long-range policies. Judging by the past week's events, that pledge too was relatively short-lived.