be a lunatic

Boyd’s Bible Dictionary:

(moon-struck). Epileptics are probably meant (Matt. 4:24; 17:15).

Concise Bible Dictionary:

These are clearly distinguished in Matthew 4:24 from those possessed by demons. The word is σεληνιαζόμενοι, which, like the word lunatic, is derived from “the moon,” and is thought to embrace epileptics as well as those of unsound mind. The lad in Matthew 17:15 is called a lunatic, but he was also possessed by a demon: in Mark 9:25 it is called a “dumb and deaf spirit.”

Strong’s Dictionary of Greek Words:

Greek:
σεληνιάζομαι
Transliteration:
seleniazomai
Phonic:
sel-ay-nee-ad’-zom-ahee
Meaning:
middle voice or passive from a presumed derivative of 4582; to be moon-struck, i.e. crazy
KJV Usage:
be a lunatic