A gentle blow. A tap on the shoulder;-an-arrest. To tap a girl; to be the first seducer: in allusion to a beer barrel. To tap a guinea; to get it changed.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1721 dictionary of canting and thieving slang.
Tantwivy * TappersNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes