Lewis & Short

formīdābĭlĭs, e, adj. [1. formido], causing fear, terrible, formidable (poet. and in post-class. prose; cf. formidolosus): lumen, Ov. M. 2, 857: nec formidabilis ulli, id. ib. 2, 174: Orcus, id. ib. 14, 116: aspectus, Gell. 14, 4, 2: sonus, id. 19, 1, 17: dolores et metus, per se formidabiles res, Sen. Ep. 123, 14.
In the neutr. adverbially: formidabile ridens, Stat. Th. 8, 582.