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4.
HSG
High School Girl.

A member of the female gender who is underage and who you would be considered a pedophile for checking out, you sick, sick bastard.
"Hey, check out that girl over there."

"You sick fuck. She's totally an HSG. I oughta report you to Child and Family Services for pederasty."
by Skippo June 10, 2004
 
1.
HSG
An abbreviation which literally refers to High School Girl(s). It can generally be applied to girls who are younger than the law would appreciate. It can be used to cover up talk which might be looked down upon by certain people in the general vicinity.
"Dude did you check out that group on facebook; I Love High School Girls? I fucking love HSG's man!!!"

"Man if it wasn't for HSG's, I might have never made it out of high school, or college, or graduate school, or my mid-life crisis."
by Principal July 07, 2006
 
2.
HSG
Stands for "Head Shaking Good", as in so awesomely amazing. Comparable to orgasm.
"Have you tried that new insert random drug, food place, drink, etc.?"
"Naw, what is it?"
"You have to try it, it's so HSG!!!"
by ricecracker7 September 26, 2011
 
3.
as in HAGS, have a good summer, someone would write this in a yearbook to mean have some good sex
hey man have a great summer and HSGS
by zachthemanissweet June 24, 2007