The full form of NFK is not f##king kidding
The acronym “NFK” is used to represent the phrase “not f##king kidding” or “no f##king kidding.” It is used interchangeably and the one being used must be determined by the context of the sentence or statement it is being used in. When used to represent the phrase “not f#$%king kidding” the person is saying they are being serious and not joking around. If a person uses “NFK” to represent the phrase “no f@#$king kidding” they are usually being sarcastic or are using it as an expression of surprise regarding something that was previously stated

Origin of NFK
There is no specific origin information available regarding this phrase. However, the phrases stated above have been used as a part of everyday informal English for many years. Like most acronyms used on the internet today and in text message conversations, the popular phrases were reduced to this acronym to make them easier, quicker, and more convenient to type or text.

Other Meanings
The acronym “NFK” is also said to be an abbreviation to represent Norfolk, Virginia. This city in Virginia is said to be the Brooklyn of the state and it is how the public and tourists refer to the city. The military also uses this abbreviation for the city of Norfolk, Virginia.

Example Conversations
A text message conversation between two friends.

Friend 1: We stayed out way too late last night. I am so tired!
Friend 2: NFK! I am exhausted! No way I am staying out that late ever again on a school night.