Green’s Dictionary of Slang

1. the human face. [it registers the emotion].

1899

J. Flynt Tramping with Tramps 271: I hain’t seen your register for many a day.

2. the vagina [abbr. cash register].

1963

in T.I. Rubin Sweet Daddy 125: The next broad leaves the works around I’ll shove a cork screw up her register.