expatiate (v.)

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.