adverb/ɑ̃.tʁə ʃjɛ̃ e lu/
between a dog and a wolf; at dusk, at twilight; gloaming; golden hour (used in same technical sense for “good photographic light at twilight”)
“Entre chien et loup, le boulevard s'est apaisé et les voitures sont en veilleuse.” — At dusk, the boulevard is quiet and the cars have their side lights on.
Calque of Latin inter canem et lupum (“between a dog and a wolf”), because at that time in the day, one can't see the difference. Traditional sentiment, dating to antiquity (found in 2nd century CE texts), in French from the 18th century.