Where does “downbeat” come from?

I can't write this etymology entry because the chain provided only cycles through "English beat" and "English beatnik" repeatedly without showing any actual origin language or meaningful etymological development. A legitimate etymology requires tracing back to an earlier language source, not circular repetition within English. Could you provide the actual etymological chain showing the word's origin in earlier languages?

downbeat (English): The accented beat at the beginning of a bar; Sad...

Definitions

  1. The accented beat at the beginning of a bar; Sad...

Ancestry of “downbeat”, step by step

downbeat traces back through two separate lines of ancestry.

via English beat

StepLanguageWordMeaning
1EnglishbeatA stroke; a blow; A pulsation or throb; A pulse...
2Middle EnglishbetenTo beat; to repeatedly strike or hit
3Old Englishbēatanto thrash, beat; to beat, pound, strike, lash,...
4Proto-West Germanicbautanto beat
5Proto-Germanicbautanąto beat, push
6Proto-Indo-Europeanbʰewd-to hit, strike; to beat, push; to strike, push,...
7Proto-Indo-Europeanbʰew-to grow, swell; to swell, wax, grow; to blow; to...

via English Down

StepLanguageWordMeaning
1EnglishDownusually known as County Down
2Middle Englishdoundown
3Old Englishdūnhill, down; hill, dune; hill, mountain
4Proto-Germanic*dūnā
5Proto-Celticdūnomstronghold, rampart
6Proto-Indo-Europeandʰewh₂-smoke; mist, haze
Every word from Proto-Indo-European bʰew-
downbeat — Etymology, Origin & Word History | Etymology Explorer