Why is Peewee called a welcher?

Peewee was called a "welcher" because he didn't keep his part of the bargain in a game they played.

We played Pitty Pat and Dirty Hearts every day. Then Monaco came up with a new game. He found some paper and put down the names of all the movie stars and singers he could think of. Then he passed them around and we played for them. That didn’t last long. Walowick won Mary Wells from Peewee and Peewee wouldn’t give her up. Walowick called Peewee a welcher.