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1.
Alternate spelling of narwhal meant to stress the gnarliness of the narwhal.
Narwhals are so gnarly that they should be named gnarwhals.
by papermachete October 28, 2005
 
2.
Superlative form of the word 'gnarly', with the progression being (absolute, comparative, superlative):

gnarly --> gnarkill --> gnarwhal

Often pronounced as 'gnarwhale'
"Dude! That Maglev car is TOTALLY gnarwhal!"
by Amerifunk December 16, 2006
 
3.
A gnarled on piece of fish leftover by someone at a restaurant, usually has the head and bones remaining.
I just cleaned up table 5 and the woman had left a nasty gnarwhal.
by JC467 June 29, 2011