Green’s Dictionary of Slang

a boxer, usu. a sparring partner.

1859		Matsell Vocabulum 128: sparring gills. See Milling Coves.
1879		‘Autobiog. of a Thief’ in Macmillan’s Mag. (London) XL 501: I [...] met a sparring bloke (pugilist) who taught me how to spar and showed me the way to put my dukes up.