Late 18th century; earliest use found in Thomas Gray (1716–1771), poet and literary scholar. From penta- + syllabic, after monosyllabic, disyllabic, trisyllabic, etc.
Definition of pentasyllabic in US English:
pentasyllabic
adjectiveˌpɛntəsɪˈlabɪk
Consisting of five syllables.
Origin
Late 18th century; earliest use found in Thomas Gray (1716–1771), poet and literary scholar. From penta- + syllabic, after monosyllabic, disyllabic, trisyllabic, etc.