-s

Meaning

(morpheme, plural) Used to form regular plurals of nouns that end in certain suffixes or syllables, such as -el, -er, -en, -em, -eur, -aar, -aard, diminutive -je, etc.
(morpheme, plural) Used to form irregular plurals of many other nouns, chiefly of foreign origin.

Unknown. Not present in Old Dutch, which used -a from Proto-Germanic *-ōz as the plural ending. Possibly spread from Middle Low German -s, -es, from Old Saxon -os, -as, from Proto-Germanic *-ōs. Further etymology is unknown, but cognate with Old English -as (English -s).