Where does “dark night” come from?

dark night (English) comes from English dark, from Middle English derk, from Old English deorc, from Proto-West Germanic derk, from Proto-Germanic derkaz, from Proto-Indo-European dʰerg-o-s, from Proto-Indo-European dʰerg-, from Proto-Indo-European dʰer- — to support; to hold.

Ancestry of “dark night”, step by step

dark night traces back through two separate lines of ancestry.

via English dark

StepLanguageWordMeaning
1EnglishdarkHaving an absolute or relative lack of light;...
2Middle Englishderkdark
3Old Englishdeorcdark
4Proto-West Germanicderkdirty; dark
5Proto-Germanicderkaz
6Proto-Indo-Europeandʰerg-o-s
7Proto-Indo-Europeandʰerg-to dim, darken; dim, dull; dark, dusky
8Proto-Indo-Europeandʰer-to support; to hold

via English night

StepLanguageWordMeaning
1EnglishnightThe period between sunset and sunrise, when a...
2Middle EnglishnyghtAlternative form of nighte; Alternative form of...
3Middle Englishnightenight
4Old EnglishnyhtAlternative form of niht
5Proto-West Germanicnahtnight
6Proto-Germanicnahtsnight
7Proto-Indo-Europeannókʷtsnight
8Proto-Indo-Europeannegʷ-bare, naked
Every word from Proto-Indo-European dʰer-Every word from Proto-Indo-European dʰerg-Every word from Proto-Indo-European dʰerg-o-s