1. To complete something very quickly and without much effort. In this usage, a noun or pronoun can be used between "whack" and "off." It used to take me quite a while to write articles for this website, but I've gotten to the point where I can whack them off in under an hour. I just need to whack off a couple emails before I head home.
2. To cut something off or away, especially quickly, indelicately, or without finesse. In this usage, a noun or pronoun can be used between "whack" and "off." I had been growing my hair out since I was in junior high school, and then one day I decided to whack it all off. The knight whacked off the peasant's arm for his insubordination. Will you go out back and whack those branches off the tree? They're obstructing our view of the sea.
3. vulgar slang To masturbate, especially a man.
4. vulgar slang To bring someone else to orgasm with one's hand or hands, especially a male. In this usage, a noun or pronoun can be used between "whack" and "off."
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
See beat off
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.