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Difficult words:
ambiguity (he presence of two or more possible meanings in a single passage.)
cliches (Formally, a cliché is “a trite, stereotyped expression; a sentence or phrase, usually expressing a popular or common thought or idea, that has lost originality, ingenuity, and impact by long overuse, as sadder but wiser, or strong as an ox.” It can also be defined as “anything that has become trite commonplace through overuse.)
monosyllabic (having only one syllable)
perception (opinion, perception, understanding)
ADDENDUM (an item of additional material added at the end of a book or document, typically in order to correct, clarify, or supplement something.)
One characteristic that will make everyone think you’re unintelligent, even if you aren’t, is speaking poorly. I don’t necessarily mean using slang or having a thick accent (although those might count against you in some circles). I mean not communicating well with words. People who speak well are usually rather precise. They say what they mean as unambiguously as possible (unless they intend ambiguity) in a way that is concise but clear.
Speaking in vague generalities and relying on cliches can make you sound less intelligent, as can an over-reliance on a small set of (often monosyllabic) words. Again, you might be smart, but you won’t come across as such.
ADDENDUM:
For some reason this answer has been getting a lot of replies lately saying that speaking well is not the same as being smart. I want to emphasize the last sentence above. This is not about whether how you speak is actually indicative of your intelligence. It is about perception. If you speak well, you can be perceived as being smarter than you are. If you speak poorly, you can be perceived as being less intelligent.