thaw

I've noticed that a lot of people say "unthaw" when really "unthawing" something would be freezing it. thawing something is to un-freeze it. The prefix "un" is to undo something well, if thaw is to un-freeze it wouldn't unthaw be to freeze it? So why on things like frozen dinners and receipes etc. does it say unthaw? It's so common now most people do say this and I guess I have too much time on my hands to actually be thinking about it but most people do say this. Even though I've thought about it I still find myself saying it. So now when you want to unfreeze something you can be sure to say "thaw" instead of "unthaw"