-most

1.
a combining form of most occurring in a series of superlatives:
foremost; utmost.
Origin
Middle English -most; replacing Middle English, Old English -mest, double superlative suffix, equivalent to -ma superlative suffix (as in Old English forma first; compare Latin prīmus) + -est1; later identified with most
British Dictionary definitions for -most

-most

suffix
1.
forming the superlative degree of some adjectives and adverbs: hindmost, uppermost
Word Origin
Old English -mǣst, -mest, originally a superlative suffix, later mistakenly taken as derived from mǣst (adv) most
Word Origin and History for -most

superlative suffix of adjectives and adverbs, from Old English -mest (altered in Middle English by influence of most), a double superlative, from -mo, -ma (cf. Old English forma "first," meduma "midmost"), cognate with Latin -mus, + -est.