Goose Gossage -- I Grew My Mustache ... To Piss Off Steinbrenner!

Yankees legend Goose Gossage tells TMZ Sports ... his legendary mustache was born out of spite -- admitting, "I actually grew it to piss [George] Steinbrenner off."

Gossage -- and his "macho tough guy mustache" -- were hangin' out at a party outside the Toyota Arcade in NYC last night ... when a QUALITY facial hair discussion broke out.

Among the highlights ...

-- He said current MLB mustaches are "a little feminine."
-- Goose calls Dennis Eckersley a "pretty boy."

But the best part ... when the Hall of Famer tells us how his legendary Fu Manchu came to be back in 1983 ... in what turned out to be anti-Yankees facial-hair policy message to the guy he once called "the fat man upstairs."