Green’s Dictionary of Slang

an act of masturbation.

1735		Rake’s Progress iii 19: Or loudly sing some bawdy Song, Then drops into St. D—n’s C—h, And take a Toss-off in the Porch [OED].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Toss off, manual pollution.
1811		Lex. Balatronicum.
1845		Rowlandson Pretty Little Games [illus.] The Toss-Off [...] She takes the circumsised part, / And plies her hand with easy art.
2000		N. Griffiths Grits 58: Might see meself off with another toss like.