The headword is the present active infinitive of the late compound verb periautologe/w (which specific form is unattested), I speak about myself; see generally LSJ s.v. If it is extracted from somewhere specific, C3-4 instances include Porphyrius, Epistula ad Anebonem 1.4b.1, and Iamblichus, de mysteriis 2.10.4.
The related abstract noun periautologia is attested somewhat earlier, notably in Plutarch; besides numerous instances in the Moralia, note his attribution of an immoderate degree of this to Cicero (Comp.Cic.Dem. 2).
[1] The headword is identically glossed in Photius' Lexicon (pi642 Theodoridis) and the Synagoge. The gloss at Lexica Segueriana 337.18 is the same except for the infinitive, which is lalei=n: to prattle about oneself.