"with a positive mood," 1947, apparently from on the upbeat "improving, getting better," attested from 1934 and a favorite of "Billboard" 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).
实用例句
1. Neil's colleagues say he was actually in a joking, upbeat mood.
内尔的同事们说他实际上在说笑逗乐，心情不错。
来自柯林斯例句
2. Scientists remain upbeat about the information that will be gathered.
科学家对即将收集到的信息仍满怀希望。
来自柯林斯例句
3. The Defense Secretary gave an upbeat assessment of the war so far.
国防部长对目前的战事给予了乐观的评价。
来自柯林斯例句
4. The tone of the speech was upbeat.
这次讲话的语气颇为乐观。
来自《权威词典》
5. The Chancellor's upbeat message that the Government had licked inflation for good was marred by more job losses.