The terms and conditions apply to copying, distribution and modification, wherein you can do what the fuck you want to.
So the WTFPL states that the copy, the distribution or the modification are subject to whatever you want to do to them. Therefore you can relicense or possibly† transfer copyright of your copy. The original is still owned by the author and is still licensed under the WTFPL. Yes, those are the same bits, even possibly traceable to the same origin. Only their Color changed[0].
† barring lawful statements that mandate copyright transfer to be explicit.
Copyright is something the comes with ownership and is what author of a work normally has; and copyright is what gives him exclusive rights with regards to copying, distributing, modifying, performing, etc. Note that copyright itself is not one of these exclusive rights granted by copyright! That would be circular.
So when you place a license on your work, you are just granting people these rights which would normally be exclusive to you as the owner, granted to her by copyright. Again: copyright is not one of these exclusive rights, copyright is property the author owns. When people receive a copy, they do not receive ownership or copyright.[2] They receive a physical copy of a work but the "intellectual property" is still held by the author. At no point was it transferred. And you cannot on your own claim and transfer the author's ownership of the property any more than you can take his home or car or wife. What you can do (as the license permits) is to practice these rights such as copying, performance, etc. which are normally exclusive to the owner.
Re-assigning or transferring copyright (in the sense that you become the owner and can sue the original author as Alupis intended) quite literally means transferring one's property to someone else. If the author of a work transfers his copyright, he no longer owns the work. It gets a new owner, just like your car would do if you gave it to someone else. Transfer of copyright is not at all the same as granting other people the same rights you have. In the US, such exclusive transfer of copyright cannot happen without black-on-white and the signature of the owner:
[1]
Any or all of the copyright owner’s exclusive rights or any subdivision of those rights may be transferred, but the transfer of exclusive rights is not valid unless that transfer is in writing and signed by the owner of the rights conveyed or such owner’s duly authorized agent.
[2]
Mere ownership of a book, manuscript, painting, or any
other copy or phonorecord does not give the possessor
the copyright. The law provides that transfer of ownership
of any material object that embodies a protected work
does not of itself convey any rights in the copyright.
EDIT: Furthermore, under the Berne Convention, original authors have the moral right to be known as the author. This right is inalienable: you can't give it away, and it cannot be taken away; you have that right even after everything else (i.e. the economic rights) is exclusively transferred to someone else. I think that makes Alupis' idea of claiming someone's code as his own work illegal.
So, in short, no, you can't re-assign copyright to yourself and then sue the original author for copyright infringement. No, you cannot claim to be the original author and to own the work. No, you do not become the owner of the work when you receive a copy along with a license that allows you to further copy, modify, distribute, perform, etc.
Under which jurisdiction? Can you back that claim up?