Merriam-Webster's Word of the Day for August 10, 2009 is:

expatiate • \ek-SPAY-shee-ayt\  • verb
1 : to move about freely or at will : wander 2 : to speak or write at length or in detail

Examples:
The middle schoolers grew restless as Mr. Donald expatiated on Pluto's classification as a dwarf planet.

Did you know?
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."

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