"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.
superlative suffix of adjectives and adverbs, Middle English alteration (by influence of unrelated most) of Old English -mest, a double superlative, from -mo, -ma (cognate with Latin -mus; compare Old English forma "first," meduma "midmost") + superlative ending -est. Now generally mistaken as a suffixal form of most.
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Definitions of outermost
situated at the farthest possible point from a center;