Pi-ra-nh: n. Any of several small, voracious fishes that in shcools attack any animal, including man . . . with strong jaws and very sharp teeth. -- Webster's New World Dictionary

By any rule of reason, the stage was nicely set only a week or so ago for at least some progress in the Mideast "automony talks."

Israel, never more beleaguered, had good reason to seek to escape the spoiler's role. Egypt's President Anwar Sadat, never more isolated in the Arab world, needed to prove he could win something for the Palestinian cause. Jimmy Carter desperately needed to win something. And the autonomy arrangements for the West Bank and Gaza are the essential next step in his Camp David master plan for peace in the Mideast.

Yet suddenly the talks hang by a thread. The "Camp David process" is now in real danger of total collapse.

"The tragedy is that we were just getting down to serious bargaining on real issues," says one American negotiator." Everybody seemed to have an interest in making it work in the next few months."

What went wrong? One explanation has it that it was never destined to go right. Israel's Prime Minister Menachem Begin, some experts insist, has never intended to permit autonomy to work -- at least not on his watch. Jimmy Carter, it is said, wouldn't apply the necessary pressure in an election year. And, anyway, Camp David was fatally flawed from the start, with Jordan and the Palestinian moderates on the outside, looking in.

But when you look at how it went wrong, and where, it isn't as simple as that. And, in a way, it's also more ominous. At every wrong turning on the way to the current impasse there is a common element. Almost invariably, you find an institution, organization or coalition uniquely vulnerable to what might be called piranha politics -- samll fish, with very sharp teeth, chewing on the loners or the weak.

In the Senate, there's President Carter and "Billygate." In the Democratic Party, there is candidate Carter and the "open convention" crowd. In both, the consequences are crippling.

At the United Nations, there's the "Third World" (with a partial assist from some of this country's supposed best friends) ganging up on Israel with a resolution so recklessly irresponsible that it doesn't even recognize Israel's right to exist. Sadat swims in a sea of Arab hate and hostility.

Finally, there's the Israeli Knesset and Begin, the supposed heavy in the breakdown of the autonomy talks. But here again, it isn't that simple. On the contrary, my nomination for Piranha of the Year is Geula Cohen, a Knesset member who cut her teeth, so to say, as a Stern Gang terroirist and bared them so demonstratively on the night the Knesset debated the Camp David accords that she had to be hauled off the floor.

Cohen is the author of the recently passed Knesset law that formally lays claim to all of Jerusalem including East Jerusalem, as Israel's capital for eternity. Having voted against the Camp David accords, she introduced the bill at a delicate moment when the autonomy talks had broken off and just as Sadat last May was informing Egypt's parliament of his willingness to start them up again. Her clear purpose was sabotage.

And it almost worked.

Only Carter's personal intervention kept the talks alive. The belief was (and still is) that Begin was prepared to disown the bill. American officials so advised Sadat.

But it turned out that Begin, in Charles de Gaulle's phrase, was too weak to bend. When the opposition Labor Party routinely supported the measure, he could not be outdone. Accordingly, Sadat felt all the more compelled to push his side of the Jerusalem argument: Jerusalem has to be at least negotiable for the sake of the future autonomy rights of some 110,000 Arabs living in East Jerusalem.

Sadat's insistence, in turn, fed Knesset support for the Cohen bill as a way of foreclosing the Jerusalem issue once and for all -- and early in the game. Israeli officials concede this also accounts for Begin's "aggressiveness" in moving his offices to East Jerusalem -- another thumb in the eye to Sadat.

And so the bill that only Geula Cohen and a few like-minded extremists really wanted to come to a vote was overwhelmingly passed, and the Camp David process has reached yet another, and perhaps even a fatal, impasse. Meanwhile, the United Nations is on record with a Nov. 15 deadline for Israeli withdrawal from "all occupied territories," and threatening to enforce it with economic sanctions and/or Israel's expulsion from the General Assembly.

In other words, the comforting wait-'til-next-year approach may not be good enough. The empty European initiative offers no alternative to the Camp David formula. There is, in short, no relief in sight from the free play of piranha politics.