Green’s Dictionary of Slang

1. to masturbate; thus whack-silly adj., addicted to masturbation and as such, mad.

1970		(con. 1950s) H. Junker ‘Fifties’ in Eisen Age of Rock 2 (1970) 103: Back with the guys, who had probably been wacking (jacking) off, beating the meat, flogging the log.
1970		T. Southern Blue Movie (1974) 11: What did they care about a handful of red-neck religious-nut hunkies (‘They’re whackin’ off to it, fer Chrissake!’).
1972		(ref. to late 1950s) B. Rodgers Queens’ Vernacular 116: whack-silly (late ’50s) 1. addicted to self abuse 2. (fr the belief that consistent masturbation causes mental deterioration) crazy. odd.
a.1973		B. Jackson Get Your Ass in the Water (1974) 226: Then I wacked off — jacked off! — and fucked the other two!
1974		(con. 1960s) R. Price Wanderers 151: Perry debated whether to whack off or not.
1986		C. Hiaasen Tourist Season (1987) 174: And now, she thought, you can all go back to the Hyatt and whack off.
1994		G. Pelecanos Shoedog 13: Bruce [Springsteen] was on the cover of both Time and Newsweek on the rack. Mal and Gary [...] teased Constantine about ‘waxing off’ on Bruce’s photo.
1994		G. Indiana Rent Boy 9: He [...] whacked himself off until the prick was bone-hard.
1997		C. Hiaasen Lucky You 49: His mother would be ruined if her only son got fired for whacking off on the job.
1998		‘Bill E. Goodhead’ Nubile Treat 🌐 Little Nell was multiorgasmic, so there was no reason for her not to whack off.
2000		Guardian Weekend 22 Jan. 32: He is a wilfully disillusioned adman [...] who whacks off in the shower.
2006		C. Hiaasen Nature Girl 92: Is he allowed to whack off once in a while?
2023		D. Whish-Wilson I Am Already Dead 168: ‘Could’ve had a heart attack while whacking off’.

2. in attrib. use of sense 1.

2015

S. King Finders Keepers (2016) 209: You don’t look all right. You’re pale and you got those dark whack-off circles under your eyes.

3. see whack v.¹ (7)

4. see whack down v. (1)