word-forming element meaning "outside; beyond the scope of; in addition to what is usual or expected," in classical Latin recorded only in extraordinarius, but more used in Medieval Latin and modern formations; it represents Latin extra (adv.) "on the outside, without, except," the old fem. ablative singular of exterus "outward, outside," comparative of ex "out of" (see ex-).
"of or pertaining to a husband, or to marriage as it pertains to the husband," hence, more broadly, "pertaining to or relating to marriage, matrimonial," c. 1600, from French maritale and directly from Latin maritalis "of or belonging to married people," from maritus "married man, husband," which is of uncertain origin (see marry (v.)).
<a href="https://www.etymonline.com/word/extramarital">Etymology of extramarital by etymonline</a>
Harper, D. (n.d.). Etymology of extramarital. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/extramarital