Entry 1 sense Webster, 1913 Argulus · Ar·gu·lus · IPA 01 n. A genus of copepod Crustacea, parasitic of fishes; a fish louse. See Branchiura. 1. A genus of copepod Crustacea, parasitic of fishes; a fish louse. See Branchiura.(Zool.) See: Branchiura