look up any word, like fleek:
 
1.
Not quite an erection, but getting there.
When I saw Milla Jovovich's boobies I got a semi.
by Germ March 15, 2003
 
2.
the state of a penis which is not altogether flaccid nor erect. It will often occur at inopportune embarrasing times and will be brought on purposefully by a cunning sexy woman pretending she has no idea what she is doing.
"why are you still friends with some of your ex-boyfriends?"

"I like to keep them healthy and providing the regular semi keeps the circulatory system in check."
by MintyLeila June 18, 2005
 
3.
the in-between state of a limp dick and a usable hard-on. can be on the way to hard, on the way back from hard, or never allowed to get more than 60% (+/-) hard.
many a young man learn to hide their semi with text books after day-dreaming in class
by the mac al February 28, 2006
 
4.
The word that comes up on t9 phones when the word "penis" is intended to be typed.
I got a text from Benjy that says, "I put my semis in her butt."
by Danielwritesstuff January 14, 2009
 
5.
Semi's is semi losers... Like semi's and full blown
Now we cross the line from semi's to full blown

I hate those fuckin semi's

by Allan S November 21, 2005
 
6.
A small boner over something none sexually related. It will be about 50% smaller
100% on a maths test forced him to get a semi
by Ben Ski Don May 03, 2010
 
7.
The act of fisting your partner with 18 fingers, being 18 fingers for 18 wheels, generally putting 6 in the front, and 12 in the back, often requiring more than 1 person giving the Semi.
I hooked up with Sammy the other day, and she really wanted to bang, but 2 of my other friends wouldn't stop tagging along where we went, so she took my offer for a good ol' Semi, I hadn't seen her again until the obituaries in the newspaper, she died from being too loose, releasing her organs out of her body.
by xXKokOfJusticeXx June 14, 2013