Brainless comes from brain + -less. Brain descends from Old English brægen ('brain'), from Proto-Germanic *bragną, of uncertain further origin. The historical trail runs through brægen in Old English and *bragną in Proto-Germanic. Morphologically, brainless is transparent: it draws on brain + -less. Used as an adjective, brainless now means "foolish or stupid; lacking intelligence", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in derived forms such as brainlessly and brainlessness and related forms such as brain and brainy. That uncertainty is part of the entry: the safest account is the attested form and the limited comparisons preserved with it.