A.T. Robertson Commentary Luke 9:39

"and behold, a spirit taketh him, and he suddenly crieth out; and it teareth him that he foameth, and it hardly departeth from him, bruising him sorely." — Luke 9:39 (ASV)

Suddenly (εξεφνης). Old adverb, but in the N.T. only in Luke's writings save Mr 13:36. Used by medical writers of sudden attacks of disease like epilepsy.

It teareth him that he foameth (σπαρασσε αυτον μετα αφρου). Literally, "It tears him with (accompanied with, μετα) foam" (old word, αφρος, only here in the N.T.). From σπαρασσω, to convulse, a common verb, but in the N.T. only here and Mr 1:26; 9:26 (and συνσπαρασσω, Mr 9:20). See Mr 9:17; Matthew 17:15; Luke 9:39 for variations in the symptoms in each Gospel. The use of μετα αφρου is a medical item.

Hardly (μολις). Late word used in place of μογις, the old Greek term (in some MSS. here) and alone in Luke's writings in the N.T. save 1 Peter 4:18; Romans 5:7.

Bruising him sorely (συντριβον αυτον). Common verb for rubbing together, crushing together like chains or as a vase . See on Matthew and Mark for discussion of details here.