Why does Sher Khan call the wolves bush-tailed thieves?

Sher-Khan is referring to the fact the wolves have taken Mowgli in and refuse to give him up.

The tiger’s roar filled the cave with thunder. Mother Wolf shook herself clear of the cubs and sprang forward, her eyes, like two green moons in the darkness, facing the blazing eyes of Shere Khan.

“And it is I, Raksha [The Demon], who answers. The man’s cub is mine, Lungri—mine to me! He shall not be killed. He shall live to run with the Pack and to hunt with the Pack; and in the end, look you, hunter of little naked cubs—frog-eater—fish-killer—he shall hunt thee! Now get hence, or by the Sambhur that I killed (I eat no starved cattle), back thou goest to thy mother, burned beast of the jungle, lamer than ever thou camest into the world! Go!”

Father Wolf looked on amazed. He had almost forgotten the days when he won Mother Wolf in fair fight from five other wolves, when she ran in the Pack and was not called The Demon for compliment’s sake. Shere Khan might have faced Father Wolf, but he could not stand up against Mother Wolf, for he knew that where he was she had all the advantage of the ground, and would fight to the death. So he backed out of the cave mouth growling, and when he was clear he shouted:

“Each dog barks in his own yard! We will see what the Pack will say to this fostering of man-cubs. The cub is mine, and to my teeth he will come in the end, O bush-tailed thieves!”