A.T. Robertson Commentary Matthew 17:15

"Lord, have mercy on my son: for he is epileptic, and suffereth grievously; for oft-times he falleth into the fire, and off-times into the water." — Matthew 17:15 (ASV)

Epileptic (σεληνιαζετα). Literally, "moonstruck," "lunatic." The symptoms of epilepsy were supposed to be aggravated by the changes of the moon (cf. 4:24).

He has it bad (κακως εχε) as often in the Synoptic Gospels.