While running with his big boots on, Tom ran into a Stone-troll eating the shinbone and skins of his nuncle Tim. The troll had been gnawing on the bones for several years. Tom tried to convince the troll to return the bone, mentioning that Tim had been gone for many years. He mentioned it should have been in a graveyard (Barrow-downs?). The troll admitted that he stole the bones from a hole, and it was okay for him to take them because Tim had been dead before he found the shinbone. Tom was upset that the troll would take the bone without asking first, for the shin of his father's kin. He asked for the troll to return the bone, but the troll threatened Tom, that if he didn't leave, he'd eat him as well (that meat would be better than bone). He attempted to capture Tom, but Tom slipped away behind him and kicked him from behind (not even putting a dent into the flesh of the troll), hurting his toes in the process, and losing his boot. Tom hobbled home without his boot, or his nuncle's bones. The troll stayed behind and continued to gnaw on Tim's shin.
Behind the scenes
Published in the Adventures of Tom Bombadil. This is a story within Hobbits stories, myths and legends, and likely based on their interpretation of Tom probably being a man, but long lived, rather than a powerful force of nature. As such Tim may not have actually existed.
Note that Tim is probably a Buckland name given to Tom's nuncle, similar to Tom Bombadil (who added the name along with the ancient ones he already carried)