If you come across a word that you suspect is a slang term, but you're not sure what it means, there's no better resource than Urban Dictionary.
I made a Keyboard Maestro macro that automates the process of looking up a term. Just select the word or phrase you want to define, then activate the macro.
A summary of how it works is below, or you can grab it from this gist.
See also: Keyboard Maestro: Look Up a Shorter Synonym on Thsrs