Green’s Dictionary of Slang

1. (orig. prison) to sodomize.

1950		‘No. 35’ Argot in G. Simes DAUS (1993).
c.1972		Parramatta Jail Gloss. 6: seat, insert penis in anus. [Ibid.] 10: nephew, a boy who is being seated but pretends ‘nice man’ is his uncle [DAUS].
1982		G. Newbold Big Huey 253: seat (v) [...] 2. Sodomise.
2000–02		Macquarie Dict. 🌐 seat Prison — verb 1. to perform anal intercourse on. — noun 2. anal intercourse.

2. to knock down.

1982

G. Newbold Big Huey 253: seat (v) 1. Knock someone to the floor.