Utmost means the very greatest possible. The utmost care, of the utmost importance, do your utmost.
It is formal and appears mostly in fixed phrases, where it does the work of a strong adjective without sounding shrill. Do your utmost means do everything you can. It is a superlative of out, and the older form was uttermost.
1adjectiveutmostThe very greatest possible.
“Please handle these old photographs with the utmost care.”
utmost is a degree.
Old English utemest, a double superlative of ut, out. Utter in the sense of complete is the same root.
Proto-Indo-European
*meh₂-
Proto-Indo-European
*úd — out, outward
Proto-Germanic
*ūt — out, outward
Old English
ūtmest
Middle English
utmost
English, today
utmost
Built from
From Middle English utmost, utemestotemeste, otemost, ottemeste, ottemoste, vttemest, vtmest, (Northern England) vtmast, (Scotland) vtmast, vtmest, from Old English ūtmest, ūtemest(Northumbria) wutmest, from ūt, ūte (“out; outdoors, outside”) + -mest (and conflated with most). Ūt is derived from Proto-Germanic ūt (“out, outward”), from Proto-Indo-European úd (“out, outward”). Equivalent to out + most.
Etymology from Wiktionary, CC BY-SA