Old English utera, uterra, "outer, exterior, external," from Proto-Germanic *utizon (source also of Old Norse utar, Old Frisian uttra, Middle Dutch utere, Dutch uiter-, Old High German uzar, German äußer "outer"), comparative adjective from ut (see out (adv.)). Meaning "complete, total" (i.e. "going to the utmost point") is from early 15c.
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.