From tabach; properly, a butcher; hence, a lifeguardsman (because he was acting as an executioner); also a cook (usually slaughtering the animal for food) -- cook, guard.
KJV: And Joseph was brought down to Egypt; and Potiphar, an officer of Pharaoh, captain of the guard, an Egyptian, bought him of the hands of the Ishmeelites, which had brought him down thither.
NASB: Now Joseph had been taken down to Egypt; and Potiphar, an Egyptian officer of Pharaoh, the captain of the bodyguard, bought him from the Ishmaelites, who had taken him down there.