[PeterPrinciple]

“plumb-bob”

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The “plumb” in “plumb-bob” comes from the fact that such tools were originally made of lead (Latin: plumbum, French: plomb). The adjective “plumb” developed by extension, as did the noun “aplomb,” from the notion of “standing upright.”

so where does the “bob” in “plumb-bob” come from?

[PeterPrinciple]

A bob is the weight on the end of a pendulum found most commonly, but not exclusively, in pendulum clocks.

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and there is the answer

[Red Badger]

It would ‘bob’ up and down when you dropped it when it reached the length of its cord?...............

[Red Badger]

https://en.wikipedia.org/wiki/Bob_(physics)

[Empireoftheatom48]

Give me a plumb, Bob.