owe, lack

欠 depicts a yawning man. Air will be blown out when someone yawns, his body will lose some air, so 欠 has the meaning of lack. On the other hand, 欠 also means the action of bowing since a person needs to blow out som air if he wants to bow. In the past, 欠 also meant the action of blowing, but such meaning had been replaced by 吹.