Shellfire is the exploding of shells — the big metal bombs — when large guns fire them. It is a war word. A town under shellfire is being hit by artillery. It has nothing to do with seashells, and it is not a kitchen fire or fireworks. “Fire” here means the shooting, as in gunfire. Though actually, the word names the attack as a whole: the bursts, the noise, the danger over a stretch of time, not one single shell falling. You will find it in news and history. It sounds formal and grim; people do not use it for a small fight with ordinary guns.