late 15c., from adverb (late 14c.), from Old French perpendiculer, from Latin perpendicularis "vertical, as a plumb line," from perpendiculum "plumb line," from perpendere "balance carefully," from per "thoroughly" (see per) + pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin"). As a noun from 1570s. Related: Perpendicularly; perpendicularity.
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