The Latin antecedent of "expatiate" is "exspatiari," which combines the prefix ex- ("out of") with "spatiari" ("to take a walk"), itself from "spatium" ("space" or "course"). Exspatiari means "to wander from a course" and, in the figurative sense, "to digress." But when English speakers began using "expatiate" in 1538, we took "wander" as simply "to move about freely." In a similar digression from the original Latin, we began using "expatiate" in a figurative sense of "to speak at length." That's the sense of the word most often used these days, usually in combination with "on" or "upon."
the naturalist is known for her willingness to expatiate on any number of issues relating to wildlife and the environment
Recent Examples on the WebWith wit and elan to spare, Greene expatiates on the intrigue that ensues when David Sparsholt, an engineering student with a fiancée, Connie, and a plan to join the Royal Air Force, arrives at Oxford in 1940. Priscilla Gilman, BostonGlobe.com, 9 Mar. 2018
Word History
Etymology
Latin exspatiatus, past participle of exspatiari to wander, digress, from ex- + spatium space, course