Green’s Dictionary of Slang

1. foolish, silly.

1929		H.C. Witwer Yes Man’s Land 22: ‘Don’t be sill!’ I exclaims.
1959		W. Brown Teen-Age Mafia 17: Don’t be sill, Ed.
1962		R. Dougherty Commissioner 276: Oh, don’t be sill.
1972		B. Rodgers Queens’ Vernacular 65: foolish, silly [...] sil.

2. of a male/female homosexual, infatuated.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1972		B. Rodgers Queens’ Vernacular 65: foolish, silly [...] sil [...] 181: sil [...] 2. (dated, les sl) female infatuated with another woman.