brainish(adjective) adjective1. characterized by undue haste and lack of thought or deliberationSimilar word(s): incautious, hotheaded, impetuous, impulsive, madcap, tearawaySentences with brainish as an adjective:- And, in this brainish apprehension, kills/ The unseen good old man. Hamlet IV. i. ca. 1602.