Green’s Dictionary of Slang

toss-off n.

also toss
[toss (off) v.]

an act of masturbation.

[UK]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].
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Toss off, manual pollution.
[UK]Lex. Balatronicum.
Rowlandson Pretty Little Games [illus.] The Toss-Off [...] She takes the circumsised part, / And plies her hand with easy art.
[UK]N. Griffiths Grits 58: Might see meself off with another toss like.