E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A whining manner of speech; class phraseology, especially of a religious nature (Latin, canto, to sing, whence chant). It is often derived from a proper name. We are told that Alexander and Andrew Cant maintained that all those who refused the Covenant ought to be excommunicated, and that those were cursed who made use of the prayer-book. These same Cants, in their grace before meat, used to pray for all those who suffered persecution for their religious opinions. (Mercurius Publicus, No. ix., 1661.)
The proper name cannot have given us the noun and verb, as they were in familiar use certainly in the time of Ben Jonson, signifying professional slang, and to use professional slang.
The doctor here,
When he discourses of dissection,
Of vena cava and of vena porta
What does he do but cant? Or if he run
To his judicial astrology,
And trowl out the trine, the quartile, and the sextile,