Green’s Dictionary of Slang

gaycat n.

[ety. unknown; the best possibility is that senses 1 and 2 were often homosexual, and thus sense 4 which in turn may suggest a transitional point between SE gay, cheerful, and gay adj. (6); note ‘The Kid’ who appears in the eponymous The Gay-cat (1921) is not openly homosexual, although this too may be contemporary self-censorship by the author]

1. [late 19C–1950s] (US tramp, also gay-catter) a young or inexperienced tramp.

2. [late 19C–1950s] (US tramp, also cat) a hobo who only accepts occasional or seasonal work.

3. [1900s–40s] (US Und.) the junior member of a criminal gang, employed to run errands or spy out possible crimes.

4. [1920s–30s] (US tramp) a tramp’s younger, homosexual companion.

5. [1920s] (US Und.) a thief who only works when he needs money.

6. (US Und.) a safebreaker.