Green’s Dictionary of Slang

jerry rumble v.

[jerry adj.2 + rumble v.2 (1); ? rhy. sl. jerryrumble = tumble v.2 (1)]

(N.Z.) to discover, to understand.

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[NZ]Truth (Wellington) 25 Aug. 5: Mary played up to the sky and fought hard, but O’Connor clinched, and soon Mary jerryrumbled that ju-jitsu was not her long suit. [Ibid.] 1 Sept. 8: Now he jerryrumbles that the fact of the matter is that he was half-cock when he waxed so eloquent [DNZE].