English is a language which is constantly evolving, with words changing meanings over the centuries. As time goes on, speaking habits change, and words are lost, only to come back much later with a radically different meaning. Like many words in the English Language, hys is a word which has picked up two meanings with fairly major differences.
The first meaning of hys is that of an archaic spelling of the pronoun his. Hys was used primarily in the 15th century when the spellings of many words in the English Language had not been fully standardized. Largely forgotten, hys shows up in unmodernized versions of classic english literature such as the autobiography of Margery Kempe.
The second, more modern usage of hys is as a gender neutral pronoun. Much like hir or eir, hys is used in the place his and/or her. Hys is less common than other gender neutral pronouns; most people who use hys do so primarily to differentiate between two people where the gender of both parties is either unknown or irrelevant.
Hys is also used in some Germanic languages, such as Afrikaans. In these languages, hys is the pronoun used to denote possession of the male gender.