Green’s Dictionary of Slang

to ‘fine’ a first-time server on a jury a sum which is spent on drink for his colleagues; also in other contexts.

1864, 1867, 1870		Hotten Sl. Dict.
1867		M. Lemon Golden Fetters II 233: [‘Commercial gents’ were and are [...] a very exclusive body.] ‘Am I right in supposing this is your maiden journey in that line?’ ‘Correct you are, sir’ replied the Commissioner. ‘Then I am bound to inform you that it is the custom in commercial rooms to “colt” a gentleman under such circumstances [...] that is, treat every gentleman present with a pint of wine.’.
1873		Sl. Dict.