A futon is a type of bedding used in Japan. Most English speakers are familiar with that word.
We already know that “za” is Japanese for sitting or to sit. So a large cushion used for bedding should be a zafuton. The Japanese language is a language that likes to makes phonetic changes for certain sounds, however, so they changed zafuton to zabuton to make it roll off the tongue a little easier.
Still, a zabuton is not bedding but it is quite large and comfortable so that’s what they call the large mat used in zazen.
To be consistent, they should call a zafu a zabu. But here the “f” sound is OK so they go back to the original futon and contract it to “fu” so that a sitting cushion is a zafu not a zabu.