'brain-equipped' originates from Modern English, specifically the compound of 'brain' and the past participle 'equipped'. 'brain' comes from Old English 'brægen' meaning 'brain', and 'equip' (hence 'equipped') comes into English via Middle French 'equiper' meaning 'to outfit' or 'to provide.'

'brain' developed from Old English 'brægen' to Middle English and then modern English 'brain'. 'equip' entered English from Middle French 'equiper' (with the past participle 'equipped') and, in Modern English, the two elements were joined as a hyphenated compound to form 'brain-equipped'.

Initially, the components meant 'the organ inside the skull' ('brain') and 'to outfit or provide' ('equip'); combined as a compound, the phrase evolved to mean 'having a brain' and, by extension, 'possessing intelligence' in modern usage.