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.
Instrument used to measure the shit pressure in the atmosphere. Used to anticipate a shit storm and potential influx of shit hawks; whom may either spread their shit or swoop down and take humans away to the shit nest.
"Do you know what a shit barometer is bubs?" "It measures the shit pressure in the atmosphere" "Your ears will implode from the shit pressure"
When dropping barometric pressure creates a pressure differential between the inside of one's intestines and the atmosphere, resulting in the urge to poop.
That miraculous sensor situated in the anal tract that enables the operator to determine whether the chamber is loaded with gas or solid matter, before risking a fart. The Bombay barometer may give faulty readings when, for example, swamped by large quantities of beer and curry, with disastrous results.