1888, "low-cut neck of a bodice" (from 1883 as a French word in English), from French décolletage, from décolleté "low-necked" (see decollete). Hence also "exposure of the neck and shoulders" (1894).
Entries linking to decolletage
decollete adj.
of women's fashionable dress, "low-necked," 1831, from French décolleté, past participle of décolleter "to bare the neck and shoulders," from dé- "off, away from" (see de-) + collet "collar of a dress," diminutive of col "neck," from Latin collum, from PIE root *kwel- (1) "revolve, move round."
By extension, of a dressed woman, "having the neck and shoulders exposed." Middle English had an equivalent and partially nativized adjective, decoloured (mid-15c.). Not to be confused with decollate (v.), which means "to behead" (see decollation).