Genitive singular of this adjective. Same or similar glossing in Photius and other lexica (references at Photius pi1067 Theodoridis), and quoted in this form from, perhaps, late epic (there is an instance in Tryphiodorus); in any event, generated ultimately by the genitive phrase describing the sea, polufloi/sboio qala/sshs, in Homer, Iliad 1.34 (web address 1 -- and 7 other instances); cf. the scholia there.
cf. floi=sbos (with a comparable gloss) at phi 546.