1550s, "somewhat dark, not luminous, dim;" see dusk + -y (2). "The normal source of an adj. in -y is a sb.; but the substantival use of dusk is not known so early as the appearance of dusky, so that the latter would appear to be one of the rare instances of a secondary adj. ..." [OED]. Meaning "rather black, dark-colored" is from 1570s. Related: Duskily; duskiness.