[Sai]

Minus doesn't sound remotely like the answer o.O Not only is its inverse not it at all, but minu isn't a word (which I'm assuming the riddle means it should be) and there is no such thing as minusing. There is deducting, not minusing.


Err, let's see, I guess I'll think of it long as I'm in the thread...
Well, I'm 99% sure I'm wrong, but at least it fits some of it. My word will be think. Thinking is an action, and if you take off its last word it becomes thin, and the opposite of thin is fat (which is not thin). However I have no idea if think answers the very first part of the riddle, about its inverse being it.