late 14c., from utter (adj.) + -most. More recent than utmost. Compare utmost. Middle English had also uttermore (late 14c.), now, alas, no longer with us.
Entries linking to uttermost
utter adj.
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.
-most
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.
utmost adj.
Old English utmest (Anglian) "outermost," double superlative of ut "out" (see out (adv.)) + -most. Meaning "being of the greatest or highest degree" is from early 14c.