Green’s Dictionary of Slang

to have sexual intercourse.

1682		‘Joan Has Been Galloping’ in Farmer Merry Songs and Ballads (1897) II 84: Joan has been Galloping all the Town o’re, / ’Till her Bumfiddle, Bumfiddle, Bumfiddle, / Until her Bumdifiddle was wonderous sore.
1685		J. Crowne Sir Courtly Nice I i: Reputation will hang loose upon a galloping lady.
1725		View of London & Westminster (2nd part) 35/1: [in a list of prostitutes] Miss Gallop [...] was been no Truant in the Loss of her time, having been seduced at Fourteen and Issue three Children before Seventeen.
1851		Life in Boston & N.Y. (Boston, MA) 28 June n.p.: Somewhere in that street, was a ‘knight of the whip’ [...] with a married woman [...] we think Mrs H**** has galloped him too hard.