Green’s Dictionary of Slang

1. (US) one who gatecrashes parties.

1922

Edwardsville Intelligencer (IL) 14 Sept. 4/4: The Flappers’ Dictionary [...] Walk in: Young man who goes to party without being invited.

2. (US) a sexually available female; thus by ext., a prostitute.

1966		Trimble 5000 Adult Sex Words and Phrases 223: walk-in (Vulg.) A girl who habitually dresses briefly to facilitate easy intercourse; a Pushover, and in a broad sense, a Prostitute.
1972		R.A. Wilson Playboy’s Book of Forbidden Words.