Definition of SLOUGH. 1. a: a place of deep mud or mire b also slew or slue \ˈslue\ (ˈslü) : 1 (swamp) : 2 an inlet on a; river: also (backwater) : 3 a creek in a marsh or tide. flat. : 2 a state of moral degradation or spiritual. — dejection \-sloughy\ ē. adjectiveefinition of SLOUGH. 1. a: a place of deep mud or mire b also slew or slue \ˈslue\ (ˈslü) : 1 (swamp) : 2 an inlet on a; river: also (backwater) : 3 a creek in a marsh or tide. flat. : 2 a state of moral degradation or spiritual. — dejection \-sloughy\ ē. ADJECTIVE. advertisement