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1.
a person that talks expertly about something he/she actually knows nothing about

(Defined by the NPR show This American Life)
Usually occurs in a conversation when you know a little about a subject and when asked to expand upon it, you extrapolate completely unrelated nonsense.

"What are trans fatty acids?"
"Fats that have an extra hydrogen atom on it"
"What is that bad for you?"
insert modern jackass line
by Rian D Fowler August 09, 2005