Unsurprisingly, Francis Grose in his A Classical Dictionary of the Vulgar Tongue (1785) has a definition for canting. It is “preaching with a whining, affected tone, perhaps a corruption of chaunting; some derive it from Andrew Cant, a famous Scotch preacher, who used that whining manner of expression”.
Canting also had a broader meaning, “a kind of gibberish used by thieves and gypsies, called likewise pedlars’ French, the slang, etc”. Canters or The Canting Crew were “thieves, beggars, and gypsies, or any others using the canting lingo”.
It may be best, if you encountered any of this disreputable lot, to make them a cap acquaintance. These, according to Grose, are “persons slightly acquainted, or only so far as to mutually salute with the hat on meeting”. Otherwise, just run a mile.