Green’s Dictionary of Slang

monkey spank n.

[rhy. sl. = wank n.; ult. spank the monkey under spank v.1 ]

an act of masturbation; lit. and fig.

D.E. Basye ‘Culture Buzz’ WWeek.com 24 Sept. 🌐 The shoddy, 20-page booklet lists only three local venues for turning a monkey spank into a trip to the bank, and they all happen to be mere sperm-donation centers.
‘SoftWhiteUnderbelly’ Fucked Company Archive 🌐 The board is now the equivalent of a urinal wall at the mall’s video arcade. A monkey-spank for 14-year-old boys thrilled at the sight of their own cum stains on their tighty-whiteys.