look up any word, like dirty sanchez:
 
1.
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
 
2.
to have a semi erection
'i was just chillin with a semdog' 'i got a semdog at work today'
by muscle shark August 09, 2009