A distant cousin of the Good, Would, Could and Dud scale of shagability.
A should is someone you could have had sex with but didn't for whatever reason.
Ahh man, remember when we met Melanie Sykes in Manc, she was well pissed. What a should. WHAT WAS I THINKING????
When you say someone has the ability to do something, and it would be advantageous to do so; they should.
However, it usually means "you better fucking ... bitch!" just in kid-friendly tone.
Jane: You should stop drinking.
Bob: You better fucking make my dinner bitch!
Like could, only replace the c with a sh.
Dude 1: C--
Dude 2: Sh!
Dude 3: You should.
Dude 4: I don't know...
Dude 5: Do it!
Dude 1: Okay.