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3.
Colloquially it is a synonym, but technically the "Sin of Onan" for which it is named for is actually an instance of coitus interruptus.
"Onanism -- it's a hands-on job"

-Ghost, ENTER THE MATRIX
by Jay Jay Ron March 20, 2008
 
1.
to wank,
to tame the one eyed monster,
to make the cyclops cry,
man's favourite outlet,
a date with mrs palm and her five
lovely daugters,
toss yourself off,
etc etc
"now on the subject of onanism...we dont want to find you hunched double on the sofa bed pumping your fist" (league of gentlemen)
by pinkpill December 01, 2004
 
2.
from New Latin onanismus, from Onan, son of Judah (Gen 38:9),
masturbation, coitus interruptus, self-gratification.
Jon can'g get any, so he practices onanism.
by El Cid June 21, 2003
 
4.
Ejaculation outside the vagina.
I think someone did onanism in my coffee again. That sure aint milk.

Well you would know scab mouth!
by Anonymous September 17, 2003
 
5.
the act of masturbating
that boy is so onanistical
he is havin a blast with that onanism
My pito is ready for its ony time
by mexican't101 February 25, 2009
 
6.
The withdrawl method of birth control

(removeing ones penis before reaching orgasm)

-not an effective method of birth control
Lets do onanism tonight hunny
by dude August 24, 2003