Brain-storm

Mr. Hammond, while engaged in some literary work connected with problems of dietetics, health and so on, in the British Museum Reading Room, was one day seized as he describes it with a sudden brain-storm, and though not having written a line of verse since boyhood suddenly found stanzas forming in his head quicker than he could record them.

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"Next time you set out to have a 'brain-storm,' Miss Malgregor," he suggested satirically, "try to have it about something more sensible than imagining that anybody is trying to hold you personally responsible for the existence of death in the world."

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The painting was “a private brain-storm which gives no evidence that Picasso has realised the political significance of Guernica.”

Source: wiktionary

Here had been tried all sorts of murder cases, with all sorts of defenses, from self-preservation with an ax to the irresponsibility of a brain-storm.

Source: wiktionary