early 14c., from Old French usurper "to (wrongfully) appropriate" (14c.), from Latin usurpare "make use of, seize for use," in later Latin "to assume unlawfully, trespass on," from usus "a use" (see use (v.)) + rapere "to seize" (see rapid (adj.)). Related: Usurped; usurping.
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Definitions of usurper from WordNet
one who wrongfully or illegally seizes and holds the place of another;