The fiscal year is one of those bits of jargon that thrives in its own natural habitat, the corporate report or the agency budget, but is never quite at home in a living-room discussion or barroom bull-session.

But in Washington this week, the talk is that the fiscal year starting tomorrow, fiscal 1982, will be a year like no other. That, of course, is because the first round of Ronald Reagan's budget and tax cuts becomes effective on Oct. 1, and the effects, both good and ill, can then begin to be measured.

As a journalist, I am in a weak position to tramp on anyone's enthusiasm for something new. Novelty, real or simulated, is the lifeblood of our business. But what strikes me as we all prepare to enter this new fiscal year and maybe this new era is the enduring importance of one of the oldest and most basic political rules of them all: keep your commitments.

Jimmy Carter fell victim to disregarding it, as much as he fell victim to inflation, the OPEC oil squeeze and the messed-up Iranian hostage situation.

The rap on Carter, from politicians who knew him before he came to Washington, was that his handshake was sometimes something less than an ironclad guarantee. He started getting in trouble in this capital when politicians of his own Democratic Party learned, to their chagrin, that he was capable of changing his mind about tax rebates, water projects and national health programs, sometimes without bothering to inform them. By the time he was reneging on things like the neutron bomb deployment and the Iowa debate challenge, the game was up for him.

Reagan and his administration-- composed of more experienced and tested politicians--had every reason to profit from Carter's unhappy example. But, increasingly, they too seem to be falling into the same trap, with the same results. And that is more worrisome for the long run than the financial quavers that usher in the new fiscal year.

The elderly in this country, to take one example, understood Reagan to say pretty plainly in his campaign that whatever the financial problems in the Social Security system, he was not going to tamper with the commitments that had been made or the benefits that had been promised. They believed that because he had said in October 1980, that the "overriding goal" of any Social Security reform must be that "the benefits of those now receiving--or looking forward to receiving--Social Security must be protected."

But twice he has tested that faith by amending the understanding. Twice he has proposed delays or reductions in prospective benefits. Twice he has stirred a hornet's nest of criticism. And twice he has backed off.

More broadly, the president said that there would be certain social "safety-net" programs for the "truly needy" that would be "exempt from cuts." But now his budget director, David A. Stockman, says "we can never promise" such exemption for any program.

Stockman also explained last weekend that the commitments he and others, speaking for the president, made to Republicans in Congress in order to secure their votes for last summer's keystone budget and tax bills were not really commitments either.

At a luncheon with reporters, he said, "People develop expectations . . . (but) we never made any commitments." Well, that just won't wash with the Republicans who were involved, and who understood that assurances had been given about the funding of dozens of programs vital to their districts. What happens when one politician begins to doubt another's word is a swift and sickening destruction of the mutual commitments that are essential to success. Without that mutual trust, government itself becomes impossible and the course of policy unpredictable.

People inside an administration get confusing signals from the top and are no longer certain what is the real policy line. The country saw that last week in the contradictory statements from Cabinet officers and White House aides on school lunch programs and the future of revenue-sharing.

Then the congressmen who have been willing to put their necks on the line for the president retreat to their customary caution, and the chief executive finds himself without the allies he needs to win the battles on Capitol Hill. Something very like that is happening to Reagan on the new round of budget cuts.

It is not too late to halt the damage. But it is none too early to focus on the risks of reviving the credibility problem that ultimately crippled Carter. This threat is a lot more serious to the Reagan administration's future than another month of high interest rates.