Green’s Dictionary of Slang

a wager.

1878		‘’Arry on the Turf’ in Punch 29 Nov. 297/1: Went down to the Derby, old pal, and put in for a regular dip.
1886		‘Lady Kate, the Dashing Female Detective’ in Roberts et al. Old Sleuths Freaky Female Detectives (1990) 33/2: ‘A bad “dip in” for the lads, that?’ ‘Chances were dead agin’ the boys.’.