An expression used to amplify the meaning of the word "fuck", which, actually, is already used to amplify something. However, the latter doesn't really mean anything, so the amplification of something that doesn't mean anything is something that doesn't fucking mean anything.
Moreover, note that the expression itself does not mean anything either since the name of fuck is "fuck".
What in the name of fuck have you done in the bathroom ?
1. A person who could be very helpful in some ways but act like a fool by refusing to help for some stupid reasons.
2. An object used incorrectly.
3. A brilliant action performed uselessly because it didn't need to be done.
4. Anything that could be useful but is completely useless in the actual context.
1. Move your fat ass and help me carrying this piano, you usefool !
2. I can see that your toaster is useful since you only use it as a hair dryer.
3. Dr. X, your last lobotomy was so usefool. The patient only had a broken arm.