downbeatyoudaoicibaDictYouDict[downbeat 词源字典]
1876 (n.), in reference to downward stroke of a conductor's baton; 1952 (adj.) in figurative sense of "pessimistic," but that is probably via associations of the word down (adv.), because the beat itself is no more pessimistic than the upbeat is optimistic.[downbeat etymology, downbeat origin, 英语词源]