'monosyllabic' originates from Greek elements: 'mono-' meaning 'one' and 'syllabē' meaning 'that which is taken together' (a unit of sound).

'monosyllabic' comes from Greek 'monosyllabikos' (via Late Latin/Medieval Latin forms such as 'monosyllabicus') and entered English in its modern form through scholarly/technical use.

Initially it meant 'having one syllable,' and over time it has retained that primary meaning while also developing the secondary sense 'using very few words' (by extension from short sound to short speech).