'brain-dead' originates from English, specifically a compound of the words 'brain' and 'dead', where 'brain' comes from Old English 'brægen' meaning 'brain' and 'dead' comes from Old English 'dēad' meaning 'dead'.

'brain-dead' developed in modern English as a compound phrase (originally written as 'brain dead' in contexts describing the medical condition); in later informal usage it is sometimes written as the single word 'braindead'.

Initially it was used literally to mean 'the brain is dead' (medical sense); over time it acquired a figurative, informal sense meaning 'mentally unresponsive' or 'extremely dull'.