imperative (n.) Look up imperative at Dictionary.com
mid-15c., in grammar; later "something imperative" (c. 1600), from Old French imperatif in the grammatical sense (13c.) and directly from Late Latin imperativus (see imperative (adj.)). In philosophy from 1796.
imperative (adj.) Look up imperative at Dictionary.com
1520s, in grammar, from Late Latin imperativus "pertaining to a command," from imperat-, past participle stem of imperare "to command, requisition," from assimilated form of in- "into, in" (from PIE root *en "in") + parare "prepare" (from PIE root *pere- (1) "to produce, procure").