expatiate
expatiate: 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” + spatiari “to walk, spread out,” from spatium. Meaning “talk or write at length” is 1610s.
This is an unusual but precise choice of word.