'monosyllable' originates from Late Latin 'monosyllabus', ultimately from Greek 'monosyllabos', where 'mono-' meant 'single' and 'syllabē' meant 'syllable (that which is uttered together)'.

'monosyllable' came into English via Late Latin 'monosyllabus' (and related medieval forms) and was adopted into Middle English as 'monosyllable', keeping much the same form.

Initially it meant 'having a single syllable'; over time the meaning has largely remained the same, referring both to the property of having one syllable (adjective) and to a word with one syllable (noun).