1530s, "walk about, roam freely," from Latin expatiatus/exspatiatus, past participle of expatiari/exspatiari "wander, digress, wander from the way; spread, extend," from ex "out" (see ex-) + spatiari "to walk, spread out," from spatium (see space (n.)). Meaning "talk or write at length" is 1610s. Related: Expatiated; expatiating.
<a href="https://www.etymonline.com/word/expatiation">Etymology of expatiation by etymonline</a>
Harper, D. (n.d.). Etymology of expatiation. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/expatiation