The Online Etymology Dictionary unsurprisingly says brainstorm is from the combination of brain and storm.
What I want to know is whether or not this neologism was an intentional pun on the word rainstorm and also whether or not, when people use brainstorm today, they think of it as a pun or an independent word from rainstorm.
I, for one, used brainstorm my entire life without thinking about rainstorm until last week when, as if in a thunderous mental flash, it suddenly occurred to me. Am I just late to make an obvious connection here? Or is this news to other people as well?
Best Answer
The answer appears to be that there is no evidence that brainstorm is a pun on rainstorm. As mentioned in the comments, it was originally used to denote 'a violent transient fit of insanity', and according to Webster, that remains its primary definition.
Elaborating on the transition to its present meaning, Ken Greenwald (Fort Collins, CO - U.S.A.) quotes from the book, America in So Many Words:
As the OP has noted, Etymonline fails to provide any additional insight:
It is, however, interesting to note a curious discrepancy in the OED in its native and American English definitions for brainstorming. The former defines it as:
IOW, the primary and secondary definitions are more or less contradictory. The AE definition also lists both definitions, but in reverse order. Webster similarly lists these contradictory views:
(with 1, of course, being about transient fits of insanity.)