dipstick

n. stick used to gauge depth; stick used to check the amount of oil in a car; fool, idiot (Slang)

A dipstick is one of several devices dipped into a liquid to perform a chemical test or to provide a measure of quantity of the liquid.In British Slang dipstick is a mild pejorative term to a foolish or contemptible person
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Dipstick /dip·stick/ a strip of cellulose chemically impregnated to render it sensitive to protein, glucose, or other substances in the urine. [more]Dipstick - Community and Resources

Noun
1. a graduated rod dipped into a container to indicate the fluid level
(hypernym) gauge, gage

a person who seems to be stupid, dimwit When Todd is nervous, he acts like a dipstick - like a fool.