Plural of brainstorm; sudden violent rush of many ideas, or sessions where people generate creative solutions together.
From 'brainstorm' (1940s coinage). The plural form is simply the standard plural morpheme '-s' added to the base noun.
Doctors in the 1800s actually used 'brainstorm' to describe sudden mental disturbances—but by the 1940s advertisers completely flipped the meaning to positive creative bursts!
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