Green’s Dictionary of Slang

a shot of alcohol (when added to an otherwise non-alcoholic drink).

1860		Bell’s Life in Sydney 11 Aug. 3/4: Other topics of the day were successively brought up, and duly washed down with hot ‘spikes’.
1900		‘Doss Chiderdoss’ ‘Each Way’ Sporting Times 24 Mar. 1/4: ‘It’s not snide!’ cried Amaryllis, with strong symptoms of the ‘spike’.
1906		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:iii 157: spike, n. Alcohol, an alcoholic beverage. ‘This punch has a good spike in it.’.