Mid 16th century (in the sense 'pomegranate'): from French, alteration of Old French (pome) grenate(see pomegranate), on the pattern of Spanish granada. The bomb was so named because it supposedly resembled a pomegranate in shape.
The Old French word grenate, the root of grenade, is a shortened form of pome grenate ‘pomegranate’, literally ‘many-seeded apple’. The connection is the supposed resemblance between the shape of the bomb and that of the fruit. Early on in its history grenade could also refer to the fruit. Continuing the fruity theme, a hand grenade has, since the First World War, been informally known as a pineapple.