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To Fionn is to eat large quantities of fatty foods or unhealthy food from a fast food outlet (such as McDonalds). The general result is an obese person or a fat ass (most commonly used in reference to males). If you were Fionning, you would be eating the food.
It can be used in context as a noun and as a verb.
Hes eating like a Fionn in a McDonalds!
by Marc Whelan February 16, 2007
V. - To Fionn: to encourage extreme sexual excitement in others merely by being present within a 400m radius from Greek "Fionnus" meaning "To sexually intimidate"

N. - Fionn(sing.)(m): A man of great street credibility and respect; one who needs to beat off women with a stick to remain unraped.

ADJ. - Fionn (-icious(-ly)): Word describing the innate attractiveness of a male individual that one had never met but heard of in passing.

Male Friend: Aw man, your the Fionn fo sho. You Fionned the shit outta that joint bro.

Lady friend/ Ho: OH, MY, GOD... I don't think I've ever had a Fionn like you do something to me that so...Fionniciously..mmmm

Male Friend: Hey man, I heard one of the bitches thinks yo shit is Fionnicious
by Doctor UserName April 15, 2009