The Greek verb aphrizo means to foam, specifically to foam at the mouth. It appears twice in the New Testament, in the parallel accounts of the healing of a boy with seizures (Mark 9:18, 20), where the demonic affliction caused the child to fall to the ground and foam at the mouth. The word describes a visible symptom of spiritual bondage.
The foaming boy in Mark 9 stands as the backdrop to one of Jesus' most direct teachings on faith and prayer: "This kind can come out only by prayer" (Mark 9:29). The disciples' inability to cast out the demon was not a ritual failure but a faith failure. Jesus' subsequent healing of the boy — with his father's cry "I believe; help my unbelief!" (Mark 9:24) — shows that even imperfect, struggling faith is sufficient when it reaches out to an all-sufficient Savior.