Outer

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late Middle English: from out + -er2, replacing earlier utter1.

wiktionary

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From Middle English outre, outer, outter, uttre, from Old English ūtre, ūtera, ūterra(“outer”), equivalent to out +‎ -er. Compare German äußere(“outer”), Danish ydre(“outer”), Swedish yttre(“outer”), Icelandic ytri(“outer”).

out (verb) +‎  -er(“agent suffix”)

etymonline

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outer (adj.)

"that is farther out, that is exterior or external; of or pertaining to the outside; further removed," late 14c., comparative of out (on analogy of inner), replacing by 18c. forms descended from Old English uttera (comparative of Old English ut "out") which developed into utter (adj.) and was no longer felt as connected with out. Outer space "region beyond the earth's atmosphere" is attested from 1845.