Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ex-sterno (ext-), āvi, ātum, 1, v. a. [sterno, ĕre; formed acc. to the analogy of consterno, āre], to drive beside one’s self, to terrify greatly, affright (poet. and very rare): aliquem assiduis luctibus, Cat. 64, 71: exsternata malo, id. 64, 165; Ov. M. 1, 641 (corresp. to exterrita); 11, 77; id. lb. 432: animos nostros perinde laetitia et dolor exsternat, Pacat. Paneg. 19. (Acc. to conject. of Jacobs, also in Hor. Ep. 1, 6, 11; and acc. to Lachm. also in Lucr. 4, 1022, v. ib. p. 266.)
externo, v. exsterno.