“My dear friend, clear your mind of cant. You may say to a man, “Sir, I am your most humble servant.” You are not his most humble servant. You tell a man, “I am sorry you had such bad weather and were so much wet.” You don’t care sixpence whether he is wet or dry. You talk in this manner; it is a mode of talking in Society: but don’t think foolishly.” (Johnson to Boswell, May 15, 1783)
The word cant as Samuel Johnson uses it here means “empty talk, insincere expressions of feelings or beliefs the speaker doesn’t really have.”
The OED gives various other meanings for the word cant, including:
• A pet phrase, a trick of words; esp. a stock phrase that is much affected at the time, or is repeated as a matter of habit or form.
• Phraseology taken up and used for fashion’s sake, without being a genuine expression of sentiment
• Affected or unreal use of religious or pietistic phraseology; language (or action) implying the pretended assumption of goodness or piety.
Cant is a doublet of chant. Both come from French chanter “to sing.”
In a broad sense, a “chant” is a song, but what most people think of as a “chant” is a monotonous song, not particularly melodious, and usually unaccompanied by a musical instrument. Chant can also be used as a verb: Monks chant their prayers. Magicians chant their spells.
How did a word meaning “song” or “to sing” give rise to one meaning “empty talk”?
Canting was a term applied to the sing-song whining pleas of beggars asking for charity. What the beggars said was perceived as insincere and their way of saying it was like singing. Thus the word cant in Dr. Johnson’s sense was born.
By the way, Dr. Johnson’s advice to Boswell remains as valuable to writers today as it was in 1783.