Does anyone know what the literal reference to "slow on the uptake" is and where it originated? I have only ever found similar phrases, but this one. I know its meaning.
Uptake (n.) meaning ability to understand is from the beginning of the 19th century. There might be an analogy with the later meaning that refers to a "pipe leading up from the smoke box":
"capacity for understanding, perceptive power," 1816, from up (adv.) + take (v.). Compare Middle English verb uptake "to pick or take up" (c. 1300).
Meaning "pipe leading up from the smoke box of a steam boiler to the chimney" is from 1839.