The act of hitting someone on the head or thinking intensely about something; the present participle of the verb 'brain.'
Present participle of 'brain' as a verb, adding -ing to the base form. 'Brain' as a verb means to strike the head or to focus mental effort, derived from the noun.
The verb 'to brain' (meaning to hit someone hard on the head) is now mostly seen in cartoons and comic books, but it was a real term in older literature. Language naturally forgets words for violence as society changes.
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