mid-14c., "state of being sated or satisfied; wholeness, totality, completion" (translating Latin saturitas, plenitudo), from full (adj.) + -ness. There might have been an Old English *fulnes but it has not survived in texts. Earlier was fullhead (c. 1300), and Middle English also had fulth "fullness, abundance" (early 14c.). Fullness of time (c. 1400) is Biblical, from Paul's letters.
<a href="https://www.etymonline.com/word/fulness">Etymology of fulness by etymonline</a>
Harper, D. (n.d.). Etymology of fulness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/fulness