S, s

suffix forming the possessive singular case of most Modern English nouns, its use gradually was extended in Middle English from Old English -es, the most common genitive inflection of masculine and neuter nouns (cf. dæg "day," genitive dæges "day's").

Old English also had genitives in -e, -re, -an, as well as "mutation-genitives" (cf. boc "book," plural bec), and the -es form never was used in plural (where -a, -ra, -na prevailed), thus avoiding the verbal ambiguity of words like kings'.

In Middle English, both the possessive singular and the common plural forms were regularly spelled es, and when the e was dropped in pronunciation and from the written word, the habit grew up of writing an apostrophe in place of the lost e in the possessive singular to distinguish it from the plural. Later the apostrophe, which had come to be looked upon as the sign of the possessive, was carried over into the plural, but was written after the s to differentiate that form from the possessive singular. By a process of popular interpretation, the 's was supposed to be a contraction for his, and in some cases the his was actually "restored." [Samuel C. Earle, et al, "Sentences and their Elements," New York: Macmillan, 1911]

suffix forming almost all Modern English plural nouns, gradually extended in Middle English from Old English -as, the nominative plural and accusative plural ending of certain "strong" masculine nouns (cf. dæg "day," nominative/accusative plural dagas "days"). The commonest Germanic declension, traceable back to the original PIE inflection system, it is also the source of the Dutch -s plurals and (by rhotacism) Scandinavian -r plurals (e.g. Swedish dagar).

Much more uniform today than originally; Old English also had a numerous category of "weak" nouns that formed their plurals in -an, and other strong nouns that formed plurals with -u. Quirk and Wrenn, in their Old English grammar, estimate that 45 percent of the nouns a student will encounter will be masculine, nearly four-fifths of them with genitive singular -es and nominative/accusative plural in -as. Less than half, but still the largest chunk.

The triumphs of -'s possessives and -s plurals represent common patterns in language: using only a handful of suffixes to do many jobs (e.g. -ing), and the most common variant squeezing out the competition. To further muddy the waters, it's been extended in slang since 1936 to singulars (e.g. ducks, sweets, babes) as an affectionate or diminutive suffix.

Old English single-syllable collectives (sheep, folk) as well as weights, measures, and units of time did not use -s. The use of it in these cases began in Middle English, but the older custom is preserved in many traditional dialects (ten pound of butter; more than seven year ago; etc.).

third person singular present indicative suffix of verbs, it represents Old English -es, -as, which began to replace -eð in Northumbrian 10c., and gradually spread south until by Shakespeare's time it had emerged from colloquialism and -eth began to be limited to more dignified speeches.