'bisyllable' originates from a combination of Latin and Greek elements: Latin 'bi-' (from Latin 'bis') meaning 'twice' and Greek 'syllabē' (via Late Latin 'syllaba'), where 'syllabē' meant 'a spoken unit (syllable)'.

'bisyllable' was formed in English by combining the Latin prefix 'bi-' with the Late Latin/Greek root 'syllaba'/'syllabē' and entered Modern English as the compound word 'bisyllable' to denote a two-syllable word.

Initially it meant 'having two syllables', and this core meaning has been retained in modern usage.