The girl amongst a group of girls in a social environment that tucks herself in a corner, and is completely silent and observant of everyone else. The side note typically is only friends with one girl in the group leaving every guy and maybe even a few of the girls wondering where the hell she came from. A side note rarely breaks the side note mold and becomes social.
"Bro, who is that girl Nicole brought last night?"
No idea, straight up side note.
"True. She was really side notin' it"
Pure side note, that's the 3rd time she's been around and not said shit to anybody.