's

suffix forming the genitive or 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 (such as dæg "day," genitive dæges "day's"). The "-es" pronunciation is retained after a sibilant.

Old English also had genitives in -e, -re, -an, as well as "mutation-genitives" (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]

As a suffix forming some adverbs, it represents the genitive singular ending of Old English masculine and neuter nouns and some adjectives.