A New Dictionary

The original hand grenade, shattering into fragments when exploding, must have put French soldiers in mind of a ripe pomegranate, filled with its dozens of seeds. Or perhaps it was simply a fancied resemblance between the outer shape of these two otherwise very different objects. Either way, it is from the Old French word for pomegranate, pome grenate, literally 'a seed-filled apple', that the French devised the name for the small hand-thrown bomb.

Taken from 'What's in a word' by John Kahn