Basic Guidelines For English Spellings
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Generally. Complete, entire, absolute; = "plenary".
Middle English (in an earlier sense). From Anglo-Norman plener, plenier, pleiner, plainer, planier, etc., and Old French, Middle French plenier (French plénier) full, absolute, consisting of the full number of members (in cort pleniere) from post-classical Latin plenarius complete; compare -ar.