Exactly what you think it is...
without the U
In other words "fuck"

n. The birth name of somebody who has changed their name. Most commonly attributed to trans people, but can be attributed to any person who has changed their name. (sometimes written as two words: dead name)

v. 1. To call somebody by their deadname.
v. 2. To out somebody's deadname to the public.

It means fuck, used in chatboxes or webboards (BBS) where "fuck" is censored.

its a way of saying the f word, without the u. which means its not as bad.

Used in place of the f-word but to friends or others who have parents who check their phones often. Couldn't ruin your stellar reputation. If someone asks what 'FCK' means you say, "failed child's knowledge." If asked what that means you say, "Ummmmmmmm." Add K's for emphasis.

Scenester Word ,, HellYes ,, I Swear Where Taking Over The World ,,

Fuck Is Normally Changed To either ..

Fck or Fxck ,, Depending On What Floats Your Boat.

Used On Myspace ,, Bebo ,, Blogging ,, Texting ,, BlahBlahBlah ,, =/

This is an acronym used in chatboxes that stands for "Forget Crying Kids"