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