n. a stupid oaf. (Also a rude term of address.) : Say, dumbo, could you move out of the way?
n. someone with large ears. (Also a rude term of address. The name of a cartoon character elephant whose ears were large enough to fly with.) : Wow, look at that dumbo with size twenty ears!
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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[duhm-boh] 
noun, plural -bos, adjective Slang.
boʊ