Defense Secretary Caspar W. Weinberger is serving notice that he is not ready yet to play "Cap the Knife" when it comes to the defense budget.

Weinberger, in recent communications with lawmakers and in interviews with the news media, has said he is unwilling to resign himself to relative austerity for the rest of President Reagan's term.

The Office of Management and Budget, in its latest word on the Pentagon's future, foresees defense budgets increasing by 3 percent plus inflation for the next five years, an assumption some lawmakers consider too optimistic. Even if the administration succeeds in reaching that level, it will be less than half of the average annual increase during Reagan's first term.

Weinberger, whose tight-fistedness when he ran the budget office gained him the nickname Cap the Knife, told a group of editors meeting with him in the Pentagon Monday that "we haven't acquiesced" to reducing future defense budgets. During his reign at the Pentagon, Weinberger's enthusiasm for ever-larger defense budgets has earned him the sobriquet "Cap the Ladle."

Congress has approved a resolution calling for the Pentagon budget to be frozen at the fiscal 1985 level, plus inflation, for fiscal 1986 and grow by 3 percent plus inflation for fiscal 1987 and 1988.

"There is no way in the world the president can say I'm going to ignore the world situation" and "make all of these decisions right now" regarding future military budgets, Weinberger said.

The military services have been told to anticipate slower growth in the next few years and to trim their weapons' wish lists substantially. A Weinberger deputy said the defense secretary has seen "the handwriting on the wall" about more modest Pentagon budgets in the future but is loathe to capitulate without protest -- particularly since defense hardliners in Congress are seeking reassurance that he has not surrendered in the fight.

Pentagon spokesman Robert B. Sims said the Pentagon is working on a fiscal 1987 budget that calls for an increase of 3 percent plus inflation, compared with the annual increases of about 7 percent in Reagan's past defense budgets.

Weinberger, in his talk with the editors, was not defying the congressional guidance for fiscal 1987 and 1988 but rather indicating that "if there's an opening for more funds, he'll go for it," Sims said.

The latest White House budget projections continue the 3 percent increases through 1990. Weinberger is scheduled to submit a report to the White House on Dec. 1 on what the impact of congressional budget cuts would be on the military.

The House and Senate Appropriations committees are considering legislation that is expected to be rougher on the Pentagon than the Armed Services committees have been in their separate process of setting ceilings on how much can be appropriated for various military functions.

In contrast to the zero growth plus inflation approved during the authorization process, the Appropriations committees are expected to freeze the fiscal 1986 budget at the 1985 level, without allowing for inflation.

"The only question," said Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, "is whether it will be zero or zero plus inflation" unless there is an international crisis in the next few years.

Weinberger, attempting to row against the changing tide in Congress, is warning that deep reductions would bring back the flawed military of the 1970s.

"In the last 18 months," Weinberger wrote Sen. Mark O. Hatfield (R-Ore.), chairman of the Senate Appropriations Committee, "we have seen the planned fiscal 1986 budget authority for the Department of Defense cut $55 billion -- a reduction of 15 percent." Further reductions "could lead us back to into the situation of the late 1970s with a military estabishment clearly unable to meet the nation's commitments."

A growing number of lawmakers hunting for ways to reduce the federal deficit see the defense budget as a large target. However, there is so much money in the pipeline from past appropriations that military spending will continue to rise for the rest of the decade even if the lawmakers vote only enough money to cover inflation through 1990.

Under zero-plus-inflation appropriations, defense spending would increase from $265.9 billion in fiscal 1986 to $318.8 billion in fiscal 1990, according to projections by the Congressional Budget Office and the Senate Budget Committee.