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."