A hand motion that signifies to someone of the opposite sex (or of the same sex depending on your sexual preference) that you are interested in them.
How to Chin Flirt: Put your hand out, four fingers extended, thumb tucked in. Then place hand palm-down under the chin. Waggle your fingers while giving come-hither looks.
Pam: Oh my God, I'm in love with Matt.
Cameron: I think he likes you too, I totally saw him chin flirt with you earlier.