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"capacity for understanding," 1816, from up + take. Cf. obsolete verb uptake "to pick or take up," attested from c.1300.
uptake up·take (ŭp'tāk')n. The absorption by a tissue of a substance, such as a nutrient, and its permanent or temporary retention.
In the brain; mentally: became a little balmy upstairs (1932+)
kick someone upstairs, the man upstairs