"ostentatiously superior and condescendingly favorable," by 1806, present-participle adjective from patronize. In 18c. generally in a more positive sense, "act as a patron to, support and encourage." Related: Patronizingly.
"with a positive mood," 1947, apparently from on the upbeat "improving, getting better," attested from 1934 and a favorite of Billboard magazine headline-writers in the early 1940s, from the musical noun upbeat (1869), referring to the beat of a bar at which the conductor's baton is in a raised position; from up (adv.) + beat (n.). The "optimistic" sense apparently for no other reason than that it sounds like a happy word (the musical upbeat is no more inherently "positive" than any other beat).
1731, "pronounce with an accent," from Medieval Latin accentuatus, past participle of accentuare "to accent," 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"). Figurative meaning "emphasize, place an accent or emphasis on" is recorded from 1865.
You've got to accentuate the positive
Eliminate the negative
Latch on to the affirmative
Don't mess with Mister In-Between
["Ac-Cent-Tchu-Ate the Positive," 1944, music by Harold Arlen, lyrics by Johnny Mercer]
Related: Accentuated; accentuating.