The headword adjective is in the accusative singular. Quoted from Hipponax (iota 588), it is transmitted variously as borbo/ropin (or borboro/pin) and, as here, borboro/phn. The eta form is orthodox (so e.g. LSJ s.v.), i.e. with the second half of the compound as o)ph/ "hole"; a technical case can be made for the iota form (see mu 1470, at n.6 there), which would require the translation "filth-faced", but the glossing seems to tell against that.
For borboros see in any event Henderson 192 #414.
[1] Literally "garden". For kh=pos in this sense (III in LSJ s.v.) see generally Henderson 135-6 #130.