To measure the depth of something, or to examine something deeply; also, to install plumbing.
From Latin 'plumbum' (lead metal). Originally, plumbers used a lead weight on a string called a plumb line to check if walls were straight or measure water depth. The word evolved from the tool to mean the action itself.
The origin is metal—Romans used lead weights to check if buildings were vertical, and we still call that a 'plumb line' today! Ironically, we've learned lead is poisonous, but the measuring technique named after it has survived thousands of years of building.
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