Origin and history of pentameter

"1 more than four; the number which is one more than four; a symbol representing this number;" Old English fif "five," from Proto-Germanic *fimfe (source also of Old Frisian fif, Old Saxon fif, Dutch vijf, Old Norse fimm, Old High German funf, Gothic fimf), from PIE root *penkwe- "five." The lost *-m- is a regular development (compare tooth).

Five-and-ten (Cent Store) is from 1880, American English, with reference to prices of goods for sale. Five-star (adj.) is from 1913 of hotels, 1945 of generals. Slang five-finger discount "theft" is from 1966. The original five-year plan was 1928 in the U.S.S.R. Five o'clock shadow attested by 1937.

[under picture of a pretty girl] "If I were a man I'd pay attention to that phrase '5 O'Clock Shadow.' It's that messy beard growth which appears prematurely about 5 P.M." [advertisement for Gem razors and blades in Life magazine, May 9, 1938]

also metre, "poetic measure, metrical scheme, arrangement of language in a series of rhythmic movements," Old English meter "meter, versification," from Latin mētrum, from Greek metron "meter, a verse; that by which anything is measured; measure, length, size, limit, proportion" (from PIE root *me- (2) "to measure").

The word was possibly reborrowed early 14c. (after a 300-year gap in recorded use), from Old French metre, with a specific sense of "metrical scheme in verse," from Latin mētrum.

The use as a word-forming element in pentameter, tetrameter, etc., was objected to by those who saw how it misapplied the classical term.

Of late years, in modern English prosody-writing, though fortunately not universally, a most objectionable habit has grown up of calling the heroic line a "pentameter," the octosyllabic iambic a "tetrameter." This is grossly unscholarly, and should never be imitated, for the proper meaning of the terms would be ten feet in the one case, eight in the other. [George Saintsbury, "Historical Manual of English Prosody," 1911]