How Well Do You Know English Slang?
"with a positive mood," 1947, apparently from the musical noun upbeat (1869), referring to the beat of a bar at which the conductor's baton is in a raised position; 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). Expression on the upbeat "improving, getting better" is recorded from 1934.
Optimistic; encouraging; positive: They use catchy, upbeat phrases/ A triumph of upbeat pictures over the downbeat
[1947+; apparently fr the musical term upbeat, ''a beat on which a conductor raises his baton,'' but since such beats have no emotional connotations, the coiner must have seized on the general positive notion of up and taken beat to mean ''stroke, movement'']