It’s when someone, specifically a girl is getting off with their partner, a stranger etc.
Abby’s getting off with some guy in the bathroom. Her friend asks what she’s doing, and another replies “oh Abby? She’s just checking the girth.”
A q-tip is something that’s bad for you but you’ll keep doing/using it anyway.
“You shouldn’t use q-tips, they’re bad for your ears”
“I’d still q-tip it”