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To have a half erect penis.
Riding a semi is often used in public to tell someone without revealing it to everyone.
Dude, I'm riding a semi right now.

So at the party I was grinding on this chick, but I started to ride a semi and had to get out of there quick.
by Stoan October 23, 2008

Words related to Riding A Semi

boner erection half boner half chub halfsie