Green’s Dictionary of Slang

down v.1

(UK Und.) to understand.

[UK]H.T. Potter New Dict. Cant (1795).
[UK]G. Andrewes Dict. Sl. and Cant.
[UK]W. Perry London Guide.
[UK]Flash Dict.
[UK]A. Thornton Don Juan in London II 221: It was of no use to try to tip him the queer– he was down to their gammon.