A thumb is an item used in the Rocking Out quest. When Captain Brass Hand Harry gives you a broken hand, it can be "taken-apart" to form a thumb, presenting the message "You lever the thumb from this hand. The rest is useless."
The thumb is then combined with 4 other parts (brace, fingers, hand, and wrist), with a hammer and 2 bronze wires in the inventory, to smith a brass hand, which is subsequently used to gain the mark from the captain.
