Shroomerang

It leaves your hand fast and flies straight for a short moment, then slows, stops and comes back. That turnaround is the important part: from the instant it starts slowing it can hit an unlimited number of targets and it stops caring about walls. So the far end of the throw is where the weapon does its damage, and you can sink the tip into a thin wall to clip whatever is standing behind it.

Aim past a group rather than at it. Water and honey drag it down and cut the distance short, while melee speed gear throws it harder and further.

It drops from the bats in the glowing mushroom biome, which is somewhere you already visit early. Even if you never use it as your main weapon it feeds a much better three-headed boomerang later, so take one when you see it.

It glows blue in flight, which makes it a passable way to scout a dark passage. The name is mushroom and boomerang jammed together.