'multisyllable' originates from Latin and Greek elements: the prefix 'multi-' from Latin 'multus' meaning 'many', and 'syllable' from Greek 'syllabē' via Latin 'syllaba', where 'syllabē' referred to a unit of speech held together.

'multisyllable' was formed in Modern English by combining the productive Latin prefix 'multi-' with the inherited word 'syllable' (from Old French/Latin from Greek 'syllabē'), paralleling related adjectives such as 'multisyllabic'.

Initially it meant 'having many syllables', and that core meaning has remained stable into modern usage.