brown whistle

A Brown whistle is when a girl gives head after hard dirty anal

or

when a guy is giving a rim job and a girl sharts.

'I bought her dinner and she wouldn't even give me a Brown whistle

I would have gave him a brown whistle, but I didn't have any breathmints

For male : residue on mouth after lips come into contact with sharted rim.

For female : residue in/around mouth after penis is in contact with poopy anal.