Green’s Dictionary of Slang

in sado-masochistic sex, dominant, pertaining to a dominatrix.

a.1989		Women in Command 16 in Murray & Murrell Lang. Sadomasochism (1989) 61: Couple [...] seek females, Dom or Sub, Gay or Straight, for interesting meetings.
1999		Gaymart.com Queer Sl. in the Gay 90s 🌐 Dom/Sub – Dominant/Submissive. Not interchangeable with top/bottom.