The headword is an adverb; see generally LSJ s.v. It is attested as early (C5-4 BCE) as Ctesias (FGrH F26 = Plutarch, Artaxerxes 14.2 [web address 1] and kappa 2521), but, along with the rest of the present passage, it is evidently extracted from a scholion to Aristophanes (see n. 2 below). Hesychius s.v. peripli/gdhn provides it as a gloss.
[1] The glossing adverb is from Sophocles fr. 596 Radt. As depicted on some vases (Gardner, p. 252) and described in Sophocles' lost eponymous tragedy, the hero Triptolemus (cf. rho 50 gloss), sent by Demeter to teach agriculture to the Greeks, drove a chariot having two serpents coiled about the axles; Lloyd-Jones, pp. 302-3.
[2] The entry, deriving from alpha 3031 (q.v.), follows the scholia to Aristophanes, Acharnians 217 (web address 2); cf. pi 1771, pi 1779, and pi 1780. The headword itself appears neither in Aristophanes' text nor in the Sophoclean fragment. The word pli/c is a Doric equivalent of bh=ma; see LSJ s.vv.
[In her critical apparatus Adler notes that mss FV omitted this entry.]