Green’s Dictionary of Slang

chalk farm n.1

[? chalk v.1 ; or ? a simple ref. to the then isolated place outside London where such duels took place]

a duel.

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[UK]W.T. Moncrieff Tom and Jerry II v: Oh, if you apologize, I’m satisfied; otherwise nothing would have done, but Chalk Farm! pistols! half-past six! pooh!