Brainless means extremely foolish. A brainless idea, absolutely brainless, brainless behaviour.
It is an insult about a particular act rather than a settled judgement of intelligence — calling a decision brainless says nobody thought at all, which is different from saying they thought badly. It suits sporting mistakes and moments of carelessness. Note that literally brainless creatures exist and manage perfectly well: jellyfish have no brain and have lasted longer than we have.
1adjectivebrainlessExtremely foolish.
“Driving that fast in the fog was a brainless thing to do.”
synonyms: stupid, thoughtless
brainless is a quality of foolishness.
Middle English braynles is brain plus -less. Brain is Old English and has no clear relatives outside Germanic, which is unusual for a body word.
Middle English
braynles
English, today
brainless
From Middle English braynles; equivalent to brain + less.
Etymology from Wiktionary, CC BY-SA