From the second edition (1989):
†usurpator
Obs.
Also 6 -our. [ad. OF. (F.) usurpateur (14th c.), or a. late L. ūsurpātor, agent-n. f. L. ūsurp-āre: see usurp v. Cf. It. usurpatore, Pr., Sp., Pg. usurpador.]
A usurper.
1529 Rastell Pastyme (1811) 63 Tirantis and usurpatours of the empyre. 1549 Compl. Scot. ix. 79 The inglismen var violent vsurpatours of al scotland. 1654 Howell Parthenop. ii. 37 Under the Iron yoak of Usurpators.