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This is a synonym for a penis in porn films. It is derived from the ability of an experienced penis and its experienced user to determine the suitability of a vagina, breast, anus or in fact any other body part for fucking or coital relations.

Typically the barometer functions by being slapped or thrust vigorously through or against the body part under test.
"My penis is the best barometer. The best test of a good tit!"
via giphy
by Azbo The Great! March 10, 2017
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