See already under delta 152. Present participle of Attic δεδίττομαι (epic δειδίσσομαι ). Oddly enough, this verb can have the causal sense "frighten" or the intransitive sense "fear." See LSJ under δειδίσσομαι and cf. delta 149, delta 150, delta 152, delta 304.
Same entry in some other lexica, including Timaeus' Platonic Lexicon; that fact makes it very likely (if not demonstrable) that the headword is quoted from Plato, Phaedrus 245B.