Halmus was a son of Sisyphus (with brothers Glaucus, Ornytion and Thersander). His name, too, appears to be connected with the sea, deriving from halme, 'sea' (more specifically, 'sea water that has dried', 'brine').

This would make him a seaman of some kind, or one who worked by the sea. Thomas Keightley suggests that the names of all four brothers may indicate the bustle of commerce since Sisyphus was a trader at Corinth.