'barmy' originates from English, specifically the word 'barm', where 'barm' meant 'yeast' or 'froth on fermenting liquid'.

'barmy' changed from the Old English word 'beorma'/'barm' meaning 'yeast'; the adjective formed as 'barm-y' in dialects to mean 'covered with barm' and later gained a figurative sense 'foolish' or 'mad'.

Initially, it meant 'relating to or covered with barm (yeast/froth)', but over time it evolved into its current meaning of 'foolish' or 'mad'.