A rod inserted into an engine's oil tank to measure how much oil is left, or slang for a foolish person.
From 'dip' (to immerse) + 'stick' (a rod). The tool originated with early automobiles in the late 19th-early 20th century. The insult meaning came later, comparing a fool to this simple device.
The dipstick is such a perfect metaphor for a dummy—it's literally a stick used to check levels, and you're basically calling someone a dumb tool. The insult became so popular in American English that most people forget it originally referred to checking oil.
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