"consisting of five metrical feet," 1540s, from 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].
The verses we have hitherto examined may be constructed at pleasure of any kind of metre—dactyl, troche, iamb, or anapest. But all at once, we now find this liberty of choice refused. We may write a pentametre verse in iambs only. A most notable phenomenon, significant of much more than I can at present understand,—how much less explain .... [Ruskin, "Elements of English Prosody, for use in St. George's Schools," 1880]