Green’s Dictionary of Slang

a halfpenny.

1821		Egan Life in London (1869) 312: Vy, it ain’t worth a single tonic.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
1886		W. Newton Secrets of Tramp Life Revealed 9: Thonics ... Half-pence or half-pennies.