In the 1938 Cary Grant & Katherine Hepburn film "Bringing Up Baby" there is a scene where Grant's character resorts to wearing Hepburn's very frilly/fuzzy bathrobe because she sent his clothes out to be dry cleaned. Someone visits while he's wearing it and asks why. Grant ad-libbed the reply "I just went gay all of a sudden." It is the first film use of close to the modern sense of the word. It had started being used in within the gay community in the 1920s, but wasn't commonly known in 1938.