The headword is a feminine noun in the accusative singular; see generally LSJ s.v. forbeia/. It is evidently extracted from somewhere, perhaps from Job 40.25 LXX or later commentary thereto; see phi 585 and its principal note.
[1] The first gloss is the masculine/feminine accusative singular (and neuter nominative/accusative singular) of the adjective peristo/mios, -on (around the mouth); see generally LSJ s.v. The second gloss, a Hellenization of the Latin capistrum, is a noun in the neuter nominative/vocative/accusative; cf. kappa 340, kappa 331 (gloss), and see LSJ s.v. Same or similar glossing in other lexica; references at Photius phi264 Theodoridis. [In her critical apparatus Adler reports that ms A transmits para\ to\ sto/mion, about the mouth.]
[2] cf. Etymologicum Magnum 798.32 (Kallierges) and Etymologicum Gudianum 556.36.
[3] cf. Apollonius, Homeric Lexicon 162.1. Adler also cites Lexicon Ambrosianum 441.
[4] Not just related in sense, but in fact properly, forbh/ means trofh/, forage; see Photius phi265 Theodoridis and the other references there, inc. Etymologicum Magnum 798.33 (Kallierges) and Eustathius, Commentaries on Homer's Iliad 2.57.6-9 (van der Valk). A scholion (= D scholia) to Homer, Iliad 11.561 (web address 1), which compares the Trojan attack on Ajax to boys driving an ass from a cornfield after the animal has gorged itself on the forage, glosses the genitive case forbh=s of Homer's verse with trofh=s. [Adler (apparatus) reports that mss FV transmit *fobh\; also that ms F reads *(/omhros (Homer) instead of o(moi/ws (similarly), a substitution that was already made by ms F at phi 510 (o(/mws, likewise).]
[5] From a scholion (citing Herodian) on Aristophanes, Birds 861 (web address 2), this grammatical note appears to assert that this spelling and accentuation are preferable to other attested variants such as forbea/, forbe/a, and forbai/a ; forbei/a with paroxytone accent occurs already at kappa 340 (gloss). In the Aristophanic text, the headword is embedded in Peisetaerus' neologism e)mpeforbeiwme/non, being fitted with a (piper's) head strap (West, p. 89), which he notes with surprise of the raven aulete; see Dunbar, pp. 508-9 (and cf. phi 587).