figurative use, "dim, indistinct," is attested from 1660s; literal use, "pertaining to or resembling twilight," from 1755, from Latin crepusculum "twilight, dusk," related to creper "obscure, uncertain," from Proto-Italic *krepos "twilight," which is of uncertain origin. It is not certain whether "twilight" or "obscure" was the original sense; de Vaan writes, "there is no known root of the form *krep- from which the extant meanings can be derived."
Especially of evening twilight, but 17c.-18c. also "like morning twilight" as symbolic of imperfect enlightenment. In zoology, "flying or appearing at sunset," from 1826. An older (and lovelier-sounding) adjective form was crepusculine (1540s).