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1.
FKW
Full Kit Wanker. Used to describe a sports fan dressed in more pieces of replica team kit than is necessary or acceptable for an adult. (note that children can never be FKWs)

For example, for football (soccer to the Americans amongst us) a shirt (jersey) is acceptable shorts and socks, however, make you a Full Kit Wanker.
Joe: I was walking through town the other day and saw a total FKW.
by Lou O November 15, 2013
 
2.
FKW
Acronym used to define the lack of knowledge of a vehicle, item or person. It stands for Fuck Knows Where
Where is Fred today? FKW

Also used for runabout vans in the days before mobile phones
by Mike Strivens October 09, 2008