1560s, "plane figure with five angles and five sides," from French pentagone (13c.) or directly from Late Latin pentagonum "pentagon," from Greek pentagōnon, a noun use of the neuter of the adjective pentagōnos "five-angled," from pente "five" (from PIE root *penkwe- "five") + gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle"). The U.S. military headquarters known as the Pentagon was completed in 1942, and so called for its shape; used allusively for "U.S. military leadership" from 1945; Pentagonese "U.S. official military jargon" is by 1951. Related: Pentagonal.

In nature, pentagonal symmetry is rare in inanimate forms. Packed soap bubbles seem to strive for it but never quite succeed, and there are no mineral crystals with true pentagonal structures. But pentagonal geometry is basic to many living things, from roses and forget-me-nots to sea urchins and starfish. [Robert Bringhurst, "The Elements of Typographic Style," 1992]

"plane figure with nine sides and nine angles," 1680s, a hybrid from Latin nonus "ninth" (from novem "nine;" see nine) + ending from pentagon, etc.

"having five angles," 1650s, from Late Latin quinquangulus "five-cornered," from quinque "five" (from PIE root *penkwe- "five") + angulus "angle" (see angle (n.)). Quinquangle (n.) "pentagon" is attested from 1660s.

"five-pointed or five-angled figure, a pentagon or pentacle," late 14c., pent-angel, "a representation of a five-pointed star;" see penta- + angle (n.). In some early uses perhaps a corruption of pentacle. Related: Pentangular.

also nit-picker, "pedantic critic," by 1951, perhaps 1946, a figurative use, said to be originally military jargon; see nit (n.) + pick (v.).

Two long-time Pentagon stand-bys are fly-speckers and nit-pickers. The first of these nouns refers to people whose sole occupation seems to be studying papers in the hope of finding flaws in the writing, rather than making any effort to improve the thought or meaning; nit-pickers are those who quarrel with trivialities of expression and meaning, but who usually end up without making concrete or justified suggestions for improvement. [Collier's, Nov. 24, 1951]