Same entry in Photius, Lexicon lambda44 Theodoridis; see further below, n. 1. The headword is in the accusative case, as determined by the grammar of the opening sentence.
The base lakk-, meaning a pond, a reservoir, appears in IE in such terms as Latin lacus, English lake, and Scots/Irish loch. See associated bibliography below. For the uncompounded noun see lambda 60, lambda 61 (and for another lakk- compound lambda 59).
[1] See already kappa 214: prominent Athenian diplomat and wealthy landowner of the early and middle C5 BCE. Brill’s Encyclopedia of the Ancient World [= Neue Pauly 1.962] regards the story as an invention based on Plutarch, Aristides 5.6 where the biographer tells a version of the story and attributes the name to the 'comic poets'. Brill/Pauly takes the term as the equivalent of 'mining baron'.
[2] For Xerxes see generally xi 54. This sea-battle took place between the Greek city-states and Persia in 480 BCE in the straits between the Athenian harbor at Piraeus and Salamis, a small island in the Saronic Gulf.