mid-14c., "to go along with, properly relate to," from be- intensive prefix, + longen "to go," from Old English langian "pertain to, to go along with," which is of uncertain origin but perhaps related to the root of long (adj.). Senses of "be the property of" and "be a member of" first recorded late 14c. Cognate with Middle Dutch belanghen, Dutch belangen, German belangen. Replaced earlier Old English gelang, with completive prefix ge-.
<a href="https://www.etymonline.com/word/belongings">Etymology of belongings by etymonline</a>
Harper, D. (n.d.). Etymology of belongings. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/belongings