Jimmy Fallon -- Had Him a Blast With ... Drunken 'Grease' Sing-Along

Jimmy Fallon relived his famous send-off from "SNL" -- singing a "Grease" classic -- but this time he had a barroom full of hammered backup singers ... which just made it more awesome.

Fallon was at NYC's Marie's Crisis Cafe last weekend when most of the bar started singing "Summer Nights" ... with Jimmy leading the chorus. But check out the video -- it wasn't easy keeping the buzzed crowd on the beat ... or the right verse.

Jimmy still managed to bring down the house, just like in his final "SNL" in 2004 -- where we're guessing there was slightly less drinking than in the bar.