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1.
When a man ejaculates deep inside a woman's rectum, causing her to have a neverending sloppy bowel movement soon after.
Guy-"You know it's hard for me to stay awake after we have sex. Why'd you take so long in the bathroom, honey?"

Girl-"Because you gave me a semenema and i couldn't stop pooping, silly."

Guy-"ZZzzzzzzz...."
by gutterwater March 14, 2009
 
2.
Ejaculating into a sexual partner's ass. A true semenema is hallmarked by outrageous amounts of semen, so much so that it actually seeps out of the rectum after withdrawal of the penis
Get ready for your semenema baby!

Doctor's orders are that your wife receive 2-3 semenema's a day, in order to regulate both bowel movements and libido.
by w00tn4nn135 February 25, 2008