"in a state of decline or decay (from a former condition of excellence)," 1837, from French décadent, back-formation from décadence (see decadence). In reference to literary (later, other artistic) schools that believed, or affected to believe, they lived in an age of artistic decadence, 1885 in French, 1888 in English. Usually in a bad sense, e.g.:
Beckoning sense of "desirable and satisfying to self-indulgence" begins c.1970 in commercial publications in reference to desserts.