*kan-

late 14c., "particular mode of pronunciation," from Old French acent "accent" (13c.), from Latin accentus "song added to speech," from ad "to" (see ad-) + cantus "a singing," past participle of canere "to sing" (from PIE root *kan- "to sing").

The Latin word was a loan-translation of Greek prosōidia, from pros- "to" + ōidē "song," which apparently described the pitch scheme in Greek verse.

The meaning "effort in utterance making one syllable stronger than another in pitch or stress" is attested from 1580s; as "mark or character used in writing to indicate accent," it is recorded by 1590s. The decorative-arts sense of "something that emphasizes or highlights" is from 1972.

The soundest distinction perhaps is that "accent" refers to the habitual stress laid on a syllable in ordinary pronunciation ; "stress" to a syllable specially accented for this or that reason, logical, rhetorical, or prosodic purely. [George Saintsbury, "Historical Manual of English Prosody," 1914]

"pretentious or insincere talk, ostentatious conventionality in speech," 1709. The earliest use is as a slang word for "the whining speech of beggars asking for alms" (1640s), from the verb in this sense (1560s), from Old North French canter (Old French chanter) "to sing, chant," from Latin cantare, frequentative of canere "to sing" (from PIE root *kan- "to sing").

Century Dictionary notes the ecclesiastical use of cantus in Medieval Latin, and writes, "The word cant may thus have become associated with beggars; but there may have been also an allusion to a perfunctory performance of divine service and hence a hypocritical use of religious phrases." The sense in English expanded after 1680 to mean "the jargon of criminals and vagabonds," and from thence the word was applied contemptuously by any sect or school to the phraseology of its rival.

... Slang is universal, whilst Cant is restricted in usage to certain classes of the community: thieves, vagrom men, and — well, their associates. ... Slang boasts a quasi-respectability denied to Cant, though Cant is frequently more enduring, its use continuing without variation of meaning for many generations. [John S. Farmer, Forewords to "Musa Pedestris," 1896]