The headword is an epic/Ionic adjective in the neuter nominative/vocative/accusative singular; see generally LSJ s.v. pranh/s, -e/s (the Attic form: see pi 2208, pi 2209). It is probably extracted from its first attestation: Homer, Iliad 2.414 (web address 1), where Agamemnon prays that Zeus allow him to cast [spears or lighted missiles, presumably] headlong down Priam's halls, to set the gates afire, and to slay Hector, while felling his Trojan comrades nearby, face-down in the dust. Here Agamemnon invokes the headword again, this second time literally, but in the masculine plural form, prhne/es (see next note); cf. pi 2266, also generated from these same verses.
[1] The headword is identically glossed in Photius' Lexicon (pi1158 Theodoridis), Lexica Segueriana 347.31, and the Synagoge. Also see Hesychius s.v.; Etymologicum Magnum 687.21 (Kallierges); Apollonius, Homeric Lexicon 134.28. Adler also cites Lexicon Ambrosianum 1117. The D scholia to Homer, Iliad 2.418 (web address 1), give this gloss to the headword's nominative plural form (see previous note).
[2] This masculine and feminine form of the headword, common to several Greek dialects, is given the above gloss by ps.-Herodian Partitiones 111.12. Adler again cites Lexicon Ambrosianum 1117.