It’s a huge vertebra, part of the backbone of a North Atlantic Right Whale.
From the 17th to the 20th century, whales were hunted at sea both for their valuable fat (or blubber) and whale bone.
Special sailing ships called whalers voyaged as far as the Arctic and Antarctic to hunt for whales.
The North Atlantic right whale got its name because it was the only kind that floated on the surface after it was killed, and so in the eyes of the whalers, was the right one to hunt, because it could be easily recovered.
In Victorian times whale fat or blubber was rendered (melted down) to make whale oil for street lamps and to light homes, and the whalebone used in a wide variety of ways, including to provide the stiffening for ladies’ corsets.