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1.
when having sex with a female as the man is about to ejaculate he removes his member and smacks it with incredible force using his penis as a hammer on the women's stomach or ass blowing his load all over her.
while steve was pounding on jills vag he surprised her with a quick hammerjack!
by ap12387 April 07, 2010
 
2.
Masturbating without lotion
Tom: " Do you Gillette "
Fred: "No I Hammer Jack "
by FFBurger1 April 12, 2012