Inner arm bone

ULNA

It’s the longer of the two forearm bones, opposite the thumb side.

The clue "Inner arm bone" refers to one of the two long bones in the forearm, with the ulna being the one on the side opposite the thumb. It’s the bone that runs parallel to the radius, often called the "inner" bone because of its position when your palms face forward. The answer is straightforward—no wordplay here—just a direct anatomical reference.