'mealy-mouthed' originates from English, specifically from combining the adjective 'mealy' (from 'meal', Middle English 'mele') and 'mouthed', where 'mealy' meant 'resembling meal or flour' (later figuratively 'soft or insipid') and 'mouth' referred to speech or manner of speaking.

'mealy' developed from Middle English 'mele'/'meal' (meaning 'ground grain, coarse flour') into the adjective 'mealy' (meaning 'powdery; bland'), and the compound 'mealy-mouthed' arose in English (recorded from the 19th century) to describe a bland or indirect way of speaking.

Initially associated with the literal idea of 'meal-like' (powdery or bland), it evolved into a figurative sense meaning 'speaking in a bland, indirect, or insincere way'; that figurative sense is the modern usage.