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20.
fos
it could mean also :

Free Open Source

fosdir.com

mean

Free Open Source Directory
by peter edin August 27, 2008
 
2.
FOS
An abbreviation, meaning "fuckable-on-site". Used to classify extremely sexy people who arouse others to an immediate, uncontrollable frenzy.
Jared Leto (of 30stm) - inherently FOS. No question.
by _Jules_ December 17, 2008
 
3.
FOS
The acronym for a person who is Full of Shit. When someone is constantly telling outrageous lies, typically about themself, in order to impress others.
"Did you hear that FOS was offered a full scholarship to Harvard, but turned it down to be a UPS driver?"
by Quinn Littles May 14, 2009
 
4.
Abreviation for "Full of Shit"
You are so "F.O.S."
They are really so "F.O.S."
by Bev Lane May 29, 2006
 
5.
FOS
Never trust a used-car salesman, they're all FOS.
by Barbarella11 December 20, 2010
 
6.
Code word for full of shit to get you out of trouble. You say it to someone and they just look confused when other people might realize what it means. Can also be pronounced Fos for short.
When my teacher told me that my homework assignment I turned in was worth a D when I knew it was worth and A, I said "You're F.O.S., this work is flawless!" I got sent to the office because apparently he knew what F.O.S. meant.
by PrincessR December 06, 2009
 
7.
Acronym for Fuck On Sight. A type of individual when seen by another, one would be willing to have sex with without knowing anything about them, not even their name.
"Dude, that girl in the parking lot, her eyes, those hips, she's a definite F.O.S."
by mayej May 10, 2009