It's from "pus", usually translated as "kitty". So Pusur on English would then be something like "mr kitty" or similar.
The "ur"-part is mostly because it sounds cool, but it's also the end of names in old Norse. You still find this in Iceland, they are often named "Gudmundur" instead of just "Gudmund" as in modern Norwegian.
No idea why.
<funfact> Notice that monday is a derivative of "Moonday" one of the three days of the week given to us by the romans. In latin, moon is Luna ae and spanish is of course a romance language, notice the similarity between "luna and lunes?" Also, in French, we see another derivative with monday being "Lundi" </funfact>