Acronym for Sexually Educated Girl. The type of girl that has received a significant sexual education so that she avoid's all measures to avoid getting pregnant or catching an STD.
Dude 1: So shall we do it,
SEG: Yes, Do you have a condom? First I need to see the expiry date on your condom. Is it a Trojan or Durex, or Store brand? Where have you stored you condom, in dry warm conditions? Do you have lube? Is it water based or oil based? I think just to be sure, I better give you a blowjob with two condoms on.
Dude: Dayuuum girl, you a SEG!