'brainless' is formed from the noun 'brain' and the suffix '-less'. 'brain' originates from Old English 'brægen', and '-less' comes from Old English 'lēas' meaning 'without'.

'brain' evolved from Old English 'brægen' through Middle English forms like 'bregen'/'braien' to the modern 'brain'; the suffix '-less' comes from Old English 'lēas'. The compound 'brainless' arose in Modern English as a literal combination meaning 'without a brain' and later gained figurative use.

Initially a literal sense 'without a brain' (rare, literal usage), it evolved into the figurative modern sense 'lacking intelligence; stupid'.