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1.
'fnarr' is an exclaimation made popular by a character called Finbarr Saunders in British adult comic 'Viz'. Use immediately after hearing someone crack a double entendré for optimal impact.
"fnarr fnarr!" - Used most appropriately when someone has said something which has dual meaning, also known as a 'double entendré'.
by Chrisfu September 09, 2003
 
2.
noun, verb
When used singularly:
a slightly self-dejectory nasal exclamation, not disimilar to the common nerd or geek's 'gowoohayyay'
Used after saying something of particular goofiness e.g. well, the mass spectrometer was calibrated with the electron microscope to produce some kind of hybrid dahlia - fnarr
by Ben Ward May 23, 2004