c. 1600, from French immémorial "old beyond memory" (16c.), from Medieval Latin immemorialis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin memorialis of or belonging to memory" (see memorial (n.)). Something immemorial is ancient beyond memory; something immemorable is not worth remembering. Latin immemor meant "unmindful, forgetful, heedless."
"obstinate, unmindful of the will or wishes of others," late 15c., from self-wille "obstinate or perverse insistence on one's own desires or opinions" (mid-14c.); see self + will (n.). Old English selfwill, selfwyll meant "free will."
Self-willedness "quality or condition of being self-willed" is from mid-15c., though it is not certain whether "obstinacy" or "self-reliance" is implied.
Middle English also had an adjective self-willy (15c.), and the adverb self-willes is attested from late 12c. as "willingly, voluntarily;" late 14c. as "willfully, stubbornly."