SETA'CEOUS, a. [L. seta, a bristle.] 1. Bristly; set with strong hairs; consisting of strong hairs; as a stiff setaceous tail. 2. In botany, bristle-shaped; having the thickness and length of a bristle; as a setaceous seaf or leaflet. Setaceous worm, a name given to a water worm that resembles a horse hair, vulgarly supposed to be an animated hair. But this is a mistake.