early 13c., "truly, plainly, outspokenly," from utter (v.) + -ly (1); meaning "to an absolute degree" is late 14c., from utter (adj.)). Cf similarly formed German äusserlich. Old English uterlic (adj.) meant "external."
"speak, say," c. 1400, in part from Middle Dutch uteren or Middle Low German utern "to turn out, show, speak," from uter "outer," comparative adjective from ut "out" (see utter (adj.)); in part from Middle English verb outen "to disclose," from Old English utan "to put out," from ut (see out (v.)). Compare German äussern "to utter, express," from aus "out;" and colloquial phrase out with it "speak up!" Formerly also used as a commercial verb (as release is now). Related: Uttered; uttering.
suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).
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.
<a href="https://www.etymonline.com/word/utterly">Etymology of utterly by etymonline</a>
Harper, D. (n.d.). Etymology of utterly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/utterly