Lewis & Short

ūsurpātīvus, a, um, adj. [usurpo, II. B. 2.], wrongly used, unusual, improper, usurpative (late Lat.): species verborum, Diom. p. 389 P.; Macr. Diff. Verb. p. 2764 ib.
Adv.: ūsurpātīvē, in an unusual manner, improperly, wrongly: usurpative ait hordea, Serv. ad Verg. G. 1, 210; id. ad Verg. A. 7, 289.