ANY DOUBTS about the seriousness of the Persian Gulf crisis evaporated at the moment when U.S. Ambassador Thomas R. Pickering told the U.N. Security Council that "Iraq crossed the Rubicon."

And as tensions remained high last weekend, a U.S. Information Agency authority on disinformation, Todd Leventhal, produced a sort of recombinant, mixed-metaphor stereotype, declaring that the Iraqis were "pulling out all the punches."

When the going gets tough, the tough call up their verbal reserves -- the trite and true, the cliche'.

Pickering found his chestnut well down a row of platitudes mothballed in the Arizona desert after the Korean War, between a thrown-down gauntlet and a line drawn in the sand. As it happened, the ambassador, with his Caesarean allusion to Lewis Diuguid is a Washington Post editor. Iraq's decision to seize American hostages, was hoist by his own petard. The parsers at the State Department at that point were avoiding the noun "hostage" as too pungent. He had misspoken, and was rolled back.

Iraqi President Saddam Hussein also came up with an aged image -- but one that already was on the comeback trail thanks to Mikhail Gorbachev. Saddam wrote to his erstwhile bete noire, Iranian President Ali Akbar Hashemi Rafsanjani, that "each of us respects the rights of the other and keeps away those fishing in muddy waters on our shores."

In May, Gorbachev, sounding the only discordant note of his trip to Washington, had said the West should not try "to fish in muddy waters" as the Soviet Union sorted out its various crises. His bon mot was disseminated widely and the Russian equivalent of the "Dictionary of Cliche's" no doubt affirms his choice of "muddy waters."

But in English, Muddy Waters is a blues singer, and the phrase Gorbachev and Saddam were angling for was "to fish in troubled waters" -- from a fishermen's maxim that "fish bite better when the water is turbulent." Still, the Soviet president has shown himself forgiving of fractured Western idiom as he winds down the Cold War, and it probably is time for Washington to acknowledge that turbulent water is doubtless muddy as well. Saddam's concept of on-shore muddy water is a bit cloudy though. In April, Secretary of State James A. Baker III had exposed Gorbachev not so much to American cliche's but to the jargon that eddies along the shores of the Potomac. The two met in Moscow on the matter of limiting missiles. According to an aide to Baker, the secretary turned to Gorbachev at the climactic moment and declared:

"You got a deal and we close ALCMs, provided Tacit Rainbow is grandfathered."

And Gorbachev accepted, surrendering abjectly to America's ninny nomenclature. After that, the next walk in the woods could only come out at a gingerbread house. The Cold War that began with the thunderclap concision of Winston Churchill was ending with the code words and acronyms of America's jargon factory, the Pentagon pin-stripe complex.

What Baker was signifying was that he saw as solved the issue of how to limit Air-Launched Cruise Missiles, but only if one latecomer in that weapon category -- with the lilting name of Tacit Rainbow -- remained outside the limits of the accord.

More on Tacit Rainbow in a moment. But as for this grandfathering, Baker, a veteran of Capitol Hill, where the complexities of grandfathering are Oedipal, has come up with a clause not far from classic: He is excluding a weapon created after a given date in the proposed treaty.

The primogenitor of grandfathering, witness the Oxford English Dictionary, is "a clause in the constitutions of some Southern states exempting from suffrage restrictions the descendants of men who voted before the Civil War."

Is this jargon the apt language of diplomacy? How does it translate? Gorbachev, a grandfather himself, is not recorded as having blanched on receiving Baker's prose.

As for Tacit Rainbow, the Pentagon is quicker to use the name than to explain it. "That name probably orginates in DARPA," said a spokeswoman for the Air Force's Tactical Air Command, who, however, could not say for what DARPA stands.

A month after Baker grandfathered Tacit Rainbow, the Pentagon bagged it, causing the developer, Northrup, to stretch it out. "We are not satisfied," said Col. Andrew Vittoria, chief of the Air Force electronic combat division, offering what he called a "snap-shot in time" of the project. He then zapped a proposed electronic aircraft called Wild Weasal.

The Washington summit meeting did not rise or fall on a Tacit Rainbow, but there was the matter of the Soviets having begun to dismantle their offensive radar station at Krasnoyarsk. That radar and two others known to the Pentagon as Flat Twin and Pawn Shop had been publicized heavily before the 1987 summit as issues capable of impeding progress at the talks.

In what can now be identified as a preliminary twitch of the Soviet eyeball prior to its fullstop blink for Baker's Tacit Rainbow, negotiator Viktor Karpov was quoted as offering assurances on the "previously dismantled and destroyed radar stations Pawn Shop and . . . Flat Twin," swallowing whole those pearls of the American jargon factory.

How will it all end? The picture is still muddy, but I see Gorbachev and Bush, arm in arm on the Saudi sands, Tacit Rainbows sheathed, pulling out all the stops and pouring oil on the troubled waters of the Persian Gulf.