1640s, of spiders, "spinning a web," from Latin retiarius, from rete "a net" (see rete). From 1650s as "net-like."
In Roman history, a retiarius was a gladiator who wore only a short tunic and carried a trident and a net. "With these implements he endeavored to entangle and despatch his adversary, who was armed with helmet, shield, and sword." [Century Dictionary].