1731, from Medieval Latin accentuatus, past participle of accentuare "to accent," from Latin accentus (see accent (n.)). Originally "to pronounce with an accent;" meaning "emphasize" 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]