'baa' originates from imitative (onomatopoeic) formation in English, specifically the Middle English word 'baa', which represented the sound made by sheep.

'baa' changed from Middle English 'baa' (an imitative word reproducing the animal sound) and remained essentially unchanged into modern English as 'baa'.

Initially, it meant 'the cry or bleat of a sheep', and over time it has retained that basic meaning as the verb 'to make that sound'.