Meaning

noun a government building with five sides that serves as the headquarters of the United States Department of Defense

noun the United States military establishment

noun a five-sided polygon

originFrom Middle French pentagone, from Late Latin pentagōnum, from Ancient Greek πεντάγωνον (pentágōnon), noun use of the neuter of the adjective πεντάγωνος (pentágōnos, “five-angled”), from πέντε (pénte, “five”) + -γωνος (-gōnos, “angled”). Equivalent to penta- + -gon.

Etymology from Wiktionary, CC BY-SA 3.0