expatiate (v.) Look up expatiate at Dictionary.com
1530s, "walk about, roam freely," from Latin expatiatus/exspatiatus, past participle of expatiari/exspatiari "wander, digress," from ex- "out" (see ex-) + spatiari "to walk, spread out," from spatium (see space). Meaning "talk or write at length" is 1610s. Related: Expatiated; expatiating.