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A half boner, or semi-erection where the penis is not yet completely erect, but not completely flacid.
Dude, that super hot chick gave me a hug and before I had any idea I was sporting a mean sem-dog.

Can I get a sem-dog for my Rem-Dog. (Said when you want your friend Remy to get some ass).
by Schloss January 12, 2006
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