Question

opposite meaning of brainless

Answer 1

opposite of brainless is brainy

Answer 2

In English, SYNONYMS are the words which have similar meaning.  Synonyms are used in similar contexts.

Antonyms are the words which have the meaning opposite to the given word (in some way).

We do not use word opposite like in your question.  The correct question should be as:    What is the antonym of  the word brainless ?

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The answers are intelligent, brainy, smart ....  These are antonyms of brainless.