stg stands for 'small tits girl'. Generally used to describe a female person (or virtual character) with small-sized breasts.
The abbreviation can (relatively) safely be used among friends in public places, without raising suspicion of the person in question.
(First seen being used in source-code of some game as a reference to a female npc)
person B: That blonde? Yeah, so?
Person A: She's cute!
Person B: Dude... why do you always go mental about planks?
Guy 2- "Yea man, he got a STG"
Also associated with the more popular crowd, but not necessarily meaning that they are.
"No, she's just an STG"
"That girl over there is such an STG"
"tell me about it, she could support the whole bronzing industry single-handedly"
Members include Chad Petree for vocals and guitar, Carah Faye for voices and vocals, Jeremy Dawson for synths and basses and Mikey Martin for drums.