Green’s Dictionary of Slang

1. an insult based on attacking the subject’s family.

1872

in ‘Mark Twain’ Life on the Mississippi (1914) 462: [as spelt] Now I didn’t fear no one giving me a back cap (exposing his past life) & running me off the job.

2. a sharp or witty reply, as offered in the ritual name-calling known as ‘the dozens’.

1936		A.E. Duckett ‘Truckin ’round Brooklyn’ in N.Y. Age 14 Nov. 7/1: He’s very pleased with that backcap.
1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.