Green’s Dictionary of Slang

a foolish homosexual man or more usu. lesbian, who is infatuated with someone.

1929		M. Bodenheim Sixty Seconds 43: It was all right to be a molly and see queer things [...] if you didn’t show this weaker side to any one—the boys in particular: they’d swat you in a minute—and kept it from bouncing up all the time lest it make you an out-and-out si-ill.
1939		D. Maurer ‘Prostitutes and Criminal Argots’ in Lang. Und. (1981) 116/2: sil. A prostitute’s female lover (jail).
1965		Guild Dict. Homosexual Terms 41: sil (or sill) (adj.): A lesbian who is in love with another woman. A contraction of silly (infatuated).
1982		Maledicta VI:1+2 (Summer/Winter) 147: From them she might pick up and more to startle than identify with her sisters use words and expressions such as [...] sil (a woman silly about another, having a crush on her).