early 15c. (transitive), from Late Latin unitus, past participle of unire "to unite, make into one" (transitive), from Latin unus "one" (from PIE root *oi-no- "one, unique"). Intransitive sense from 1610s. Related: United; uniting.
<a href="https://www.etymonline.com/word/united">Etymology of united by etymonline</a>
Harper, D. (n.d.). Etymology of united. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/united