The headword is an epic epithet for Poseidon (though used by Pindar for other gods, cities, etc.). Same or similar glossing in other lexica, and see also a scholion on Homer, Iliad 8.201, where the headword occurs in the vocative (web address 1; also Iliad 7.455 and Odyssey 13.140).
Also attested as a (human) proper name; notably that of the man who, paired with Procles, was regarded as the founder of one of the Spartan royal families.