Green’s Dictionary of Slang

barnard n.

[SE berner, one who waits with a relay of hounds to intercept a hunted animal]

(UK Und.) that member of a team of swindlers who poses as an independent individual, ostensibly having no knowledge of his new companions but keenly ready to befriend the victim and often pretending to be drunk.

[UK]G. Walker Detection of Vyle and Detestable Use of Dice Play 39: While the street and company gather to the fray [...] the barnard steals away with all the stuff.
[UK]Greene Notable Discovery of Coosnage in Grosart (1881–3) X 10: As they are set, comes in the Barnard, stumbling into your companie, like some aged Farmer of the Countrey, a straunger unto you all, that had been at some market Towne thereabout, buying and selling, and there tipled so much Malmesie that he has neuer a ready woord in his mouth, and is so carelessse of his money.
W. Watson Quodlibets Religion and State 61: Practicall science inuented by fig-boyes, and men of the Bernard high lawe.
[UK]Dekker Belman of London F1: The Barnard is the chiefe Player, for hee counterfeites many parts in one, and is now a drunken man, anon in another humour, and shifts himself into so many shapes, only to blind the Cozen.

In compounds

barnard’s law (n.) [law n. (1)]

(UK Und.) a form of card-sharping in which a team of four con-men fleece a victim.

[UK]G. Walker Detection of Vyle and Detestable Use of Dice Play 37: Another oily shift, and for the subtyl invention and fitness of wit exceedeth far all the rest, is the barnard’s law: which, to be exactly practised, asketh four persons at least, each of them to play a long several part by himself.
[UK]Greene Notable Discovery of Coosnage in Grosart (1881–3) X 9: There was before this many yeeres agoe a practise pit in use by such shifting companions, which was called the Barnards law, wherein as in the arte of Cunny-catching, four person were required to perfourm their cosning commodity. The taker-vp, the Verser, the Barnard and the Rutter.
[UK]Dekker Belman of London F: Maintaining themselues onely by the cozenage they vse in Carde-playing: which kind of play of theirs, they call The Barnards Law.