Thank you, Kermit, for this valuable lesson. 

From the Latin perpendicularis, which is from perpendiculum (plumb line). which in turn came from per (through) and pendere (to hang).  It then came through Old French and was picked up by Late Middle English (in the 1500s) as an adverb meaning "at right angles". 

Late Gothic  architecture in England from the 14th to 17th centuries was based solely on perpendicular lines and angles.  This style was unique to England, because no comparable style can be found in Continental Europe or anywhere else in the British Isles.  

Euclid mentioned  parallel and perpendicular lines  over 2,000  years ago, but never formulated a complete description of the term.  That didn't happen until the 17th century when Rene Descartes came up with the idea of Cartesian Coordinates, which is used to locate a point in space by giving its relative distance from perpendicular intersecting lines.