Entry 2 senses Webster, 1913 Half-deck /(-dĕk`)/ · Half·-deck · IPA 01 n. A shell of the genus Crepidula; a boat shell. See Boat shell. 1. A shell of the genus Crepidula; a boat shell. See Boat shell.(Zool.) See: Boat shell 2. See Half deck, under Deck. See: Deck