ONLINE ETYMOLOGY DICTIONARY:pentagon
(n.)
- plane figure with five angles and five sides, 1560s, from M.Fr. pentagone or directly from L.L. pentagonum "pentagon," from Gk. pentagonon, noun use of neuter of adjective pentagonos "five-angled," from pente "five" (see five) + gonia "angle" (see knee (n.)). The U.S. military headquarters Pentagon was completed 1942, so called for its shape; used allusively for "U.S. military leadership" from 1945. Related: Pentagonal.