Green’s Dictionary of Slang

1. (US) a very small amount.

1896		H. Blossom Checkers 139: Stand a tap on the mare today. She can’t lose.
1902		Bulletin (Sydney) 22 Nov. 17/2: We have a dog, a one-eyed bull-terrier, that has never done a tap in his life.
1912		J. London Smoke Bellew (1926) 21: You’ve never earned a cent in your life, nor done a tap of man’s work.
1927		W. Edge Main Stem 23: Ain’t done a tap o’ work for fifty years.
1936		K. Mackenzie Living Rough 241: I ain’t done a tap of work in ten years.
1940		W.R. Burnett High Sierra in Four Novels (1984) 389: We’re rich. We don’t never have to do a tap of work again.
1953		H. Tracy Mind You, I’ve Said Nothing (1961) 73: In whole villages hardly a tap of work would be done till the magic fortnight was over.
1963		B. Crump ‘One of Us’ in Best of Barry Crump (1974) 141: We lived like lords for three months and hardly did a tap of work.
1975		R. Macklin Queenslander 181: Old Mavis hasn’t done a tap for anyone but Arthur.

2. (US drugs) a small amount of a narcotic drug.

2013

T. Piccirilli Last Whisper in the Dark 314: ‘Here, give me another tap of that [i.e. heroin].’ I handed it to him. He took a pinch, raised his fingers to his nostrils, and had a snort.