The word in question is a form of the word "ἅπτω", whose primary meaning is to fasten or bind something to something else. Liddell-Scott cites tying a noose to a beam, for example.
In the middle voice, as here in 1 John 5:18, the meaning shifts to a more reflexive mood, and describes someone, as it were, "fastening themselves to", or grasping something. A more modern word might be "grab". This has plenty of examples in the New Testament, such as when people try to grab the fringe of Jesus' cloak (Mt 9:20-21 and 14:36, Mk 3:10, 5:27-31 and 6:56, and Luke 6:19 and 8:44-47), but also for picking up a lamp, grabbing someone's hand, or Jesus putting his hands on someone's eyes to heal them.
One can infer that if the evil one were to grab someone, that would be harmful, but the word does not connote this by itself. Using the word "touch" is closer, but is missing the connotation of taking hold of securely.