"one who inherits, or has right of inheritance in, the property of another," c. 1300, from Anglo-French heir, Old French oir "heir, successor; heritage, inheritance," from Latin heredem (nominative heres) "heir, heiress" (see heredity). Heir apparent (late 14c.) has the French order of noun-adjective, though it was not originally so written in English. It is the heir of one still alive whose right is clear. After death the heir apparent becomes the heir-at-law. Related: Heir-apparency.
fem. suffix, from French -esse, from Late Latin -issa, from Greek -issa (cognate with Old English fem. agent suffix -icge); rare in classical Greek but more common later, in diakonissa "deaconess" and other Church terms picked up by Latin.
<a href="https://www.etymonline.com/word/heiress">Etymology of heiress by etymonline</a>
Harper, D. (n.d.). Etymology of heiress. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/heiress