loose as a dock

Is when your friend is such a slut she is called "loose as a dock" (a boating dock).

Schoolgirl 1: So how many guys have you slept with this week?
Schoolgirl 2: um... bout 15 ... I think.
Schoolgirl 1: But it's only Wednesday! You are as loose as a dock, bitch!
Schoolgirl 2: Fuck you!

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