What is Public domain?

No restrictions apply to works in the public domain, where the creator has given up ownership of and all rights to the work. They may be freely modified, and the creator of the derivative work may license any new portions of the derivative work, but not the public domain portion, under any terms, or none. The resulting derivative work may not be available to the creators of the original or may compete with them. So, a public domain work is by definition not subject to full copyleft.