Jamieson, Fausset and Brown on Mark 9:18

18. And wheresoever he taketh him, he teareth him; and he foameth, and gnasheth with his teeth, and pineth away--rather, "becomes withered," "dried up," or "paralyzed"; as the same word is everywhere else rendered in the New Testament. Some additional particulars are given by Luke, and by our Evangelist below. "Lo," says he in Lu 9:39, "a spirit taketh him, and he suddenly crieth out; and it teareth him that he foameth again, and bruising him hardly [or with difficulty] departeth from him."

and I spake to thy disciples that they should cast him out; and they could not--Our Lord replies to the father by a severe rebuke to the disciples. As if wounded at the exposure before such a multitude, of the weakness of His disciples' faith, which doubtless He felt as a reflection on Himself, He puts them to the blush before all, but in language fitted only to raise expectation of what He Himself would do.