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This is one of those times that UrbanDictionary.com is a great resource (but don't believe it all the time. Some entries are a joke--literally! People sometimes put fake entries as a joke). Fly on Urban Dictionary.
Alex Lynch and Cypherpunk/UrbanDictionary are right- fly is cool, good, fun, or attractive depending on context. I feel I should add though, that the word is no longer used much as slang in everyday conversation (at least in my area), and people who say it sound as if they're straight from the 1990's -- right off The Fresh Prince of Bel-Air