Green’s Dictionary of Slang

1. (US) to appear, to make oneself available.

1989		R. Atkinson Long Gray Line (1990) 60: You [...] to the sinks. Post!
1997		Simon & Burns Corner (1998) 125: Nor could he post for evening games when he’d be on shift at the hospital.
2006		G. Pelecanos Night Gardener 41: ‘And why ain’t you post?’ ‘’Cause I ain’t had no money.’.

2. (W.I.) to miss an appointment.

1995

Francis-Jackson Official Dancehall Dict. 41: Post (to post someone) not showing up for an appointment: u. a post de bwoy.