Green’s Dictionary of Slang

harlequin n.

[SE Harlequin, a character in English pantomime who wears parti-coloured, bespangled tights; the shininess, and thus the colours reflected in it]

(orig. theatre) a sovereign.

[UK]Barrère & Leland Dict. of Sl., Jargon and Cant.
[UK]Farmer & Henley Sl. and Its Analogues.