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1.
A member of the opposite sex with whom you would not have sex.

This is based on an ingenious recalibration of the conventional 0 - 10 scale so that 5 is the equilibrium point at which you would just have sex with a person, but not really be enthusiastic about it. The magnitude of ratings above 5 denotes how badly you want to bone, and the magnitude of ratings below 5 denotes how vehemently you'd avoid boning.

An alternate definition of a rating of 5 useful for males is describing someone which you would neither thrust into nor attempt to remove from your penis.

Recognized antonyms: Super-5, Supra-5
"I would rather brush my teeth with jizz than have sex with her, she's incredibly sub-5. She's a 2."

"Eh, I don't really care, shes exactly a 5. I'd let her climb my dick, I guess."

"I'd pay $100 to put my dick in her hair. She's a 9."
by Gil Barry March 15, 2010
 
2.
-noun, plural

1. A person/people who rank below the do-able level.

2. The people who rank 0 through 4.9 on a standard 1-10 grading scale with 10 being the best.
Q: What would you rate SJP?

A: Pft... sub5.
by herrocroser November 12, 2010