"consisting of five metrical feet," :1540s, from Middle French pentametre, from Latin pentameter, from Greek pentametros (adj.) "having five measures," from pente "five" (see five) + metron "measure" (from PIE root *me- (2) "to measure"). As a noun from 1580s, "a verse line of five feet;" in ancient prosody "a dactylic dipenthemimeres or combination of two catalectic dactylic tripodies" [Century Dictionary].