late 14c., sequestren, transitive, "remove (something), set aside; quarantine, isolate (someone); excommunicate;" also intransitive, "separate oneself from," from Old French sequestrer (14c.) and directly from Late Latin sequestrare "to place in safekeeping," from Latin sequester "trustee, mediator," noun use of an adjective meaning "intermediate," which probably is related to sequi "to follow" (from PIE root *sekw- (1) "to follow").
The legal meaning "seize by authority, confiscate" is attested from 1510s. The alternative verb sequestrate is early 15c. (Chauliac), from the Latin past participle sequestratus. Related: Sequestered; sequestering.