Definition of GRENADIER. 1. a: a soldier who carries and throws grenades b: a member of a special regiment or corps formerly armed with grenades.2. : any of various deep-sea fishes (family Macrouridae) that are related to the cods and have an elongate tapering body and compressed pointed tail.. a: a soldier who carries and throws grenades b: a member of a special regiment or corps formerly armed with grenades. 2. : any of various deep-sea fishes (family Macrouridae) that are related to the cods and have an elongate tapering body and compressed pointed tail. See grenadier defined for kids.