TAGF is short for "that ain't gonna fly", to be used in the context of somebody saying something really stupid or ludicrous.
Person 1: Hey, I saw this air guitar for sale online for $50!
Person 2: No way!
Person 1: ikr?! tagf.
Person 2: No way!
Person 1: ikr?! tagf.
by GraviPy September 6, 2013
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