The headword is a feminine noun in the nominative singular. It is a late form of forbei/a, halter for feeding (which ties a horse, for example, to a manger); cf. kappa 340 (gloss), phi 586, and see generally LSJ s.v. [In her critical apparatus Adler reports that ms F omits the entry, and ms A the glossing word.]
The basis for this entry -- and phi 586 -- may be Job 40.25 LXX ("will you put a feeding-string about [the serpent's] nose?"), which gave rise to patristic commentary.
[1] For the gloss (a Hellenization of the Latin capistrum), cf. kappa 331 (gloss), kappa 340, and see generally LSJ s.v.