As for ‘man’ ‘men’, that is a case of what in German is termed Umlaut, or “modification of vowel,” a phenomenon of wide range in Germanic language, but of which the results are reduced almost to minimum in English. It was originally the alteration of an ‘a’-sound to an ‘e’-sound by the assimilating influence of a following ‘i’ […] a change, therefore, which depended on the character of the case-ending, and had nothing whatever to do with the distinction of plural from singular […].
- Whitney (1875), Pag. 127