It may not rank with such all-time great T-shirts as the Rolling Stones' big-mouth logo, or the Frankie Goes to Hollywood wearable posters. But in this town, at this moment, the hot T-shirt is the one Georgetown coach John Thompson flashed during the St. John's-Georgetown basketball game in New York Wednesday night.

It started as a sweater, first worn by St. John's coach Lou Carnesecca, who put it on at his wife's urging because he wasn't feeling terrific before the Pittsburgh game. St. John's won that night, and Carnesecca wore the "lucky sweater" to all the games that followed, according to Katha Quinn, sports information director at St. John's.

Carnesecca was given the chevron-patterned sweater by an Italian team coach at an international clinic, Quinn says. But the fact that it isn't yet available in stores here hasn't stopped others from wearing a similar style. A New York paper printed instructions that obviously a number of people used to knit up their own.

It wasn't a sweater, however, but a screen-printed T-shirt copy that Georgetown's Coach Thompson flashed under his jacket at the Madison Square Garden match. Apparently someone bought him the screen-printed version of the brown, red and blue sweater at the St. John's bookstore. The entire arena cheered when he revealed the T-shirt before the game.

St. John's bookstore director Robert Heins says a supplier brought him a prototype of the shirt, he got Carnesecca's approval of the knockoff, and a deal was struck with the supplier. "We got 'em in, put 'em out, and away they go," says Heins, who estimates he has sold between 200 and 300 to date.

He isn't worried about a slack in sales following the St. John's loss to Georgetown. "We still have the Big East tournament to go," he says. (The T-shirt is available by mail order for $12, including tax and postage, through the St. John's University Bookstore, Jamaica, Queens, N.Y., 11439.)

The St. John's basketball team has inspired more than one T-shirt. Students at Niagara University in New York have been sporting "We beat St. John's" T-shirts since Niagara defeated the number one team earlier in the season.