Though he spare it, i.e. will not part with it; or gratify and obey it, instead of subduing and mortifying it. Keep it still within his mouth, to suck out all the sweetness or benefit of it, though it be very delightful to him.
Related Commentaries of Job 20:13
Job 20:13
13 Though he spare it, and forsake it not; but keep it still withind his mouth: