The headword phrase combines the genitive singular of the masculine noun plakou=s with the perfect middle/passive participle, masculine genitive singular, of Attic pe/ttw (Ionic pe/ssw). It is presumably quoted from somewhere, perhaps (given the context) a lost work of Aristophanes. Various other combinations of them occur at Peace 869, Ecclesiazusae 223, and fr. 211 Kassel-Austin (202 Kock). On Aristophanes’ use of these words see Henderson (below), 144. The entire entry, as Adler notes, is very similar to pi 1397 (where Apollo is in the dative, parallel with Hermes and Theseus).
[1] Aristophanes, Plutus [Wealth] 1126, with fuller scholia; cf. pi 1397. The fourth is Hermes' birthday (Homeric Hymn to Hermes 19) and the occasion of both public and private offerings to him; see Sommerstein (below), 210 ad loc. On the monthly festival days in general, see Mikalson (below), 13-24.