Green’s Dictionary of Slang

1. (US black) an occasional lover.

1928

R. Lacy ‘Ham Hound Grave’ 🎵 Let me be your sometime now until your always come.

2. (US prison) one who cannot be depended upon [? the excuse, ‘I’ll do it sometime, I’ll get round to it sometime’; cf. Sp. mañana, lit. tomorrow].

1981

Maledicta V:1+2 (Summer + Winter) 265: He also steers clear of a half-stepper, who does things half way, or a sometime, who is a person one cannot depend on.