Green’s Dictionary of Slang

to knock down; thus as v.i. to be knocked out.

1822		Morn. Chron. (London) 21 Aug. 3/4: It was one of those stultifying hits which sends an antagonist ‘dorsing’ [...] In this round (5) Parish was hit to sleep with the right hand.
1827		Egan Anecdotes of the Turf, the Chase etc. 217: Peg tipped Martha the Irish fling, which almost dorsed her.