His father warned that he would tan him with a belt if he skipped school again.

tan 在这里是动词，意思是"用皮带抽打、鞭打"，这个用法比较口语化。tan 的本义是指"鞣制皮革"或"晒黑"，在这里引申为"用皮带抽打"的意思，因为皮带通常是用鞣制过的皮革制成的。