A.T. Robertson Commentary Mark 9:18

"and wheresoever it taketh him, it dasheth him down: and he foameth, and grindeth his teeth, and pineth away: and I spake to thy disciples that they should cast it out; and they were not able." — Mark 9:18 (ASV)

Wheresoever it taketh him (οπου εαν αυτον καταλαβη). Seizes him down. Our word catalepsy is this same word. The word is used by Galen and Hippocrates for fits. The word is very common in the papyri in various senses as in the older Greek. Each of the verbs here in Mark is a graphic picture.

Dashes down (ρησσε). Also ρηγνυμι, μ form. Convulses, rends, tears asunder. Old and common word.

Foameth (αφριζε). Here only in the N.T. Poetic and late word.

Grindeth (τριζε). Another hapax legomenon in the N.T. Old word for making a shrill cry or squeak.

Pineth away (ξηραινετα). Old word for drying or withering as of grass in Jas 1:11.

And they were not able (κα ουκ ισχυσαν). They did not have the strength (ισχυς) to handle this case. See Mt 17:16; Luke 9:40 (κα ουκ ηδυνηθησαν, first aorist passive). It was a tragedy.