Frank Terpil, an international merchant of weapons and torture devices who fled his McLean home last September to avoid New York state charges of selling machine guns and federal charges of training terroists in Libya, has always had a penchant for shooting off his mouth.

In three tape-recorded meetings in Room No. 2737 of the New York Hilton in the late fall of 1979, Terpil's irrepressible gabbing helped get him sentenced last week to 53 years in prison. He was convicted in absentia in a New York state court in May of conspiring to seel 10,000 machine guns to New York City undercover agents and for possession of pistols, rifles and explosives. The tapes, introduced as evidence in his trial, also provided a chilling insider's account of Idi Amin's bloody Uganda and the ghoulish, amoral world of international arms dealing.

Terpil l41, a former CIA agent, boasted that he attended a state dinner in Kampala with Amin where the severed head of a Ugandan defense minister was presented on a large serving tray, according to tape transcripts. Terpil and his fellow arms merchant, George Gregory Korkala, also told undercover agents of poisoning an innocent patron at a Beirut restaurant, witnessing gruesome tortures in Uganda and training Libyans to turn automobiles into fragmentation bombs.

Both terpil and Korkala are believed to be in the Middle East pursuing their international arms business. Matthew Crosson, an assistant Manhattan district attorney in charge of the Terpil case, said yesterday that Terpil is probably living in Damascus, Syria, and Korkala is staying at the Weiner Haus in Beirut, a well-known meeting place for the Palestine Liberation Organization.

Edwin P. Wilson, another former CIA employe indicted with Terpil, is believed to be living in Libya, working as an arms procurer for Libyan leader Col. Muammar Qaddafi.

Terpil, according to Ugandan government documents, sold $3.2 million worth of weapons, surveillance equipment and explosives to Amin's government, beginning in 1978. Before his dealing with Amin, he allegedly worked with Wilson in providing arms and other military equipment to Libya. Terpil and Wilson also have been linked with an aborted assassination attempt of one of Qaddafi's political opponents in 1976.

Before he skipped this country, Terpil lived in a $260,000 Japanese contemporary home on Chain Bridge Road in McLean, less than a half mile from Ethel Kennedy's estate. Terpil's home, which features a basement firing range and a bookcase that opens into a network of concealed passageways, has been seized by the Internal Revenue Service in lieu of $2.8 million in unpaid taxes.

Terpil, who was born in Brooklyn, worked for the CIA as a communications specialist in the Middle East and Pakistan from 1965 to 1971. According to sources in Washington, he was fired because his superiors could not tolerate his indiscriminate boasting.

Terpil, a stocky, brown-haired man who has spent a good part of his adult life as an arms merchants shuttling between the United States, Europe and Africa, first met with undercover agents in the New York Hilton on Nov. 1979.

Terpil told two undercover agents, who were posing as South American terrorists in need of machine guns and who were known only as Raad and Rodriguez, that he was an important member of Amin's Ugandan entourage, that he was the only American allowed to witness public executions in Uganda.

According to the transcript. Terpil launched into a bizarre account of a state dinner that Amin held for his senior ministers. Terpil said that Amin ate with his hands, as was his custom.

"Yea, so he [Amin] said [to the assembled ministers], 'I know which one of you people are working for the Kenyans.' He said, 'The minister of defense told me.' Now nobody knows the minister is gone. See, he just doesn't show up for the dinner and Amin says we have him up at the State Research Bureau, which, man, that was a one-way gate. That's where my offices were, up there," Terpil says in the transcript. (Documents found in Kampala after Amin was deposed in the spring of 1979 show that Terpil helped train members of the dreaded State Research Bureau, which engaged in murder and torture.)

The Terpil transcript continues: "Amin said, 'One of you people [at the table] is working for Kenya. If you tell me we'll let bygones be bygones.' And if a guy believes that, he's got to be a freak. Nobody tells him, so they bring on the food. . . . Fried bananas. . . . Terrible. And then they bring on the meat.

"They put this big f------ tray in the middle of the table and Idi's guy [the waiter] lifts off the lid and there is the f ------ head of the minister of defense."

Terpil, according to the transcript, told agents that one minister at the table gasped. Amin, sensing he had found the traitor, fired a bullet into the gasping minister's face, Terpil said.

Terpil said that the bullet was fired from a .357 magnum pistol which he had given Amin. Engraved on the weapon was what Terpil described as Amin's favorite expression: "I want this man killed completely."

In the Hilton meetings, Terpil's colleague, Dorkala, who ran an arms manufacturing firm in Nutley, N.J., before he fled the country last fall, presented a laundry list of arms available to the supposed South American terrorists. They included American M16 rifles, Soviet AK47s, French submachine guns, ground-to-air hand-lanched rockets, laser sights and explosives.

Korkala also tried to interest the undercover agents in poisons, according to the transcript. He said he helped test, in a Beirut restaurant, the effectiveness of 50 kilos of cyanide he sold to a group of Middle Eastern terrorists.

Korkala: "When they tested [the poison], they put a spoonful in the guy's soup. . . . We didn't know who the guy was. We knew they were going to do something [laughter on the transcript] a f------ spoonful in his soup and when he took a drink . . . one spoonful and his head fell in the bowl of soup. He was gone like that." The transcript indicates that Korkala snapped his fingers.