COG. (I) A broadly built, round-shaped ship, used as a trader and also as a ship of war till the 15th century (M.E. cogge, cf. O.Fr. cogue, from which "cock-boat" is derived) . (2) A tooth in a series of teeth, on the circumference of a wheel, which works with the tooth in a corresponding series on another wheel (see MECHANICS) . A slang term for a form of cheating at dice.