forming the possessive singular of nouns and some pronouns
forming the possessive plural of nouns whose plurals do not end
contraction of us
where's he live?
what's he do?
Collins English Dictionary. Copyright © HarperCollins Publishers
Word origin of '-s'senses 1, 2: assimilated contraction from Middle English -es, from Old English, masculine and neuter genitive singular; sense 3, equivalent to