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1.
The act of ejaculating onto a female's breasts.
Dustin: So dude did you try that new thing with her tits? A rack sack?

Mitch: Oh I totally rack sacked her.
by I_AM_THE_CLIT_COMMANDER May 17, 2007
 
2.
The condition a male is forcefully given when having been struck in the testicles. This condition is known to cause severe watering of the eyes, temporary swelling of the testicles, extreme cases of Sailor Mouth, and will cause the one stricken with Racksack to assume the fetal position, most likely with their hands protecting their testicles from further harm.
Ouch, bro, that nutshot must've given you some serious Racksack.
by Sol Reaper February 10, 2011