Aptly stands for 'Super Tan Girl'; a combination of a Gina and Valley Girl into one entity. Usually blonde, but is based on the overexcessive and obvious fake bake the girl has undergone.
Also associated with the more popular crowd, but not necessarily meaning that they are.
"Do you think that she went on vacation?"
"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"