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."