Lewis & Short

portentōsus (-tŭōsus), a, um, adj. [portentum], full of monsters, monstrous, portentous, unnatural, hideous, revolting, etc. (class., but not used of abstract things till after the Aug. period): si quando aliqua portentosa aut ex pecude aut ex homine nata dicuntur, Cic. Div. 2, 28, 60: puer portentoso parvoque capite, Suet. Dom. 4: portentosissima genera ciborum, id. Calig. 37: labyrinthi, vel portentosissimum humani impendii opus, Plin. 36, 13, 19, § 84: mendacia Graeciae, id. 5, 1, 1, § 4: scientia, i. e. of wonderful things, id. 23, 1, 27, § 58: ars, id. 30, 1, 2, § 8: ingenia, id. 9, 41, 65, § 140: quo quid fieri portentosius potest? Sen. Ep. 87, 19: oratio portentosissima, id. ib. 114, 8.