Green’s Dictionary of Slang

(UK Und.) to understand.

a.1790		H.T. Potter New Dict. Cant (1795).
1809		G. Andrewes Dict. Sl. and Cant.
1818		W. Perry London Guide.
1821		Flash Dict.
1836		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.