The WTFPL is almost a public domain grant. It is super-permissive. Basically, do whatever you want. Note, however, that it is not a trademark license.
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
Version 2, December 2004
Copyright (C) 2004 Sam Hocevar [firstname.lastname@example.org]
Everyone is permitted to copy and distribute verbatim or modified copies of this license document, and changing it is allowed as long as the name is changed.
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
0. You just DO WHAT THE FUCK YOU WANT TO.
Describes the ability to use the software for commercial purposes.
Describes the ability to modify the software and create derivatives.
Describes the ability to distribute original or modified (derivative) works.
Describes the ability for you to grant/extend a license to the software.
Describes whether the original copyright must be retained.
Describes whether copies of the original software or instructions to obtain copies must be distributed with the software.
Including the full text of license in modified software.
Describes the ability to use/modify software freely without distributing it.
Describes the warranty and if the software/license owner can be charged for damages.
Describes the ability to change software's name if modified/distributed.