Lord, have mercy - The word "Lord" here means "Sir," a title of civility, not implying divinity.
My son - This was an only son (Luke). He was possessed with a devil. This calamity was attended with the following symptoms: he was lunatic (see the notes at Matthew 4:24); he was sore vexed; that is, he suffered greatly, or was greatly afflicted; he fell often suddenly, in the manner of persons having epileptic fits; he was dumb - that is, he was mute except when the seizure was coming upon him, for Luke says that when the spirit took him he cried suddenly out; he foamed and gnashed with his teeth, and wasted away, or became poor and emaciated. Luke Luke 9:39 adds of the evil spirit, "it teareth him that he foameth again, and, bruising him, hardly departeth from him;" that is, scarcely departed from him, or he had only short intervals of reason, for so the passage in Luke, "bruising him, hardly departeth from him," should be translated.

My son - is lunatic - Σεληνιαζεται. One who was most affected with this disorder at the change and full of the moon. See on Matthew 4:24 (note). But this lunacy was occasioned by a demon, see Matthew 17:18, and Mark 9:17; Luke 9:38. In this case, the devil intended to hide himself under the appearance of a natural disorder, that no supernatural means might be resorted to for his expulsion. See a remarkable account on Luke 9:39 (note).
Falleth ofttimes into the fire, and oft into the water - The paroxysms of his disorder frequently recurred; and among his numerous falls, some were into the fire and some into the water: so that, on this account, his life was in continual danger. Those who are under the influence of the devil are often driven to extremes in every thing. Such are often driven into the fire of presumption, or the waters of despair. Satan takes advantage of our natural temper, state of health, and outward circumstances, to plague and ruin our souls.

Lord, have mercy on my son: for he is (g) lunatick, and sore vexed: for ofttimes he falleth into the fire, and oft into the water.
(g) They that at certain times of the moon are troubled with the falling sickness, or any other kind of disease: but in this case, we must so understand it, that besides the natural disease he had a demonic derangement.

Lord, have mercy on my son,.... He addressed him with great marks of honour and respect, not only by gesture, but by words; he craves mercy, pity, and compassion; for the case he had to present, was a miserable one; and his earnestness and importunity he hoped might be excused, since it was for a child of his own. Luke adds "for he is mine only child"; and therefore his affection for him must be thought to be very strong, and he greatly concerned for its grievous affliction, and earnestly desirous of its health and life.
For he is lunatic: not a mad man, but troubled with the epileptic disease; upon which, as on madness or lunacy, the changes and full of the moon have an influence: hence the next clause,
and sore vexed, is rendered in the Arabic version, "and sore vexed at the beginning of full moons"; at which times, he had very grievous and frequent fits of his disorder:
for ofttimes he falleth into the fire, and oft into the water: which shows it to be the "epilepsy", or "falling sickness", he was afflicted with; which, whenever it seized him, whether by the fireside, or by the side of a river or brook, or any place of water, or in any other dangerous situation, he fell into it, not being able to help himself, or avoid any danger to which he was exposed. A larger account of this child's disorder, and of the circumstances of his cure, are related by Mark 9:17 where this case will be more fully considered. See Gill on Mark 9:17. See Gill on Mark 9:18.

My son; for he is a lunatic. Epileptic, in the Revision. The symptoms are those of epilepsy, in this case caused by demoniac possession. The son was a child (Luke 9:38). He was dumb as well as epileptic (Mark).

He is lunatic - This word might with great propriety he used, though the case was mostly preternatural; as the evil spirit would undoubtedly take advantage of the influence which the changes of the moon have on the brain and nerves.