The word "fag bag" is used to define a single pound of marijuana
As you may or may not know, a pound of marijuana
is a ridicilious amount to have at one time. So, the term originates from all the faggot
attention whores that try to look cool around your school when they shout out and brag about the absurd amounts of marijuana
they consumed the day before.
It's typically used to make fun of those types of kids in a sarcastic high-pitched tone of voice.
Attention whore: "Oh, my God! I'm still so high from yesterday after smokin' so much weed yesterday!"
Person 1: "Dude, Naithen. You don't have to yell it, we're right next to you."
:::Attention whore walks away:::
Person 1: (in a mocking tone of voice)"O, mah gaw! I sm0k3d a hug3 fag bag yest3rdaay."
Person 2: (laughs) "Yeah, I know man. He's a faggot."