A person is considered high-flown if he tries to sound smarter or fancier than he really is.
The adjective high-flown particularly comes in handy when you're describing overblown, pretentious writing. You can also describe overly lofty plans as high-flown: "The six year-old's intention to build a tree house with wall-to-wall carpeting, electric lights, and plumbing seemed a little high-flown to me."
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