unthinkable

An unthinkable thing is one that's impossible or goes against all sense and reason.

You often hear people say "that's unthinkable" when they're shocked by something someone has done. For example, if a friend cheats on an exam, you might think it's unthinkable behavior from them. You also use this word to talk about actions or situations that are just plain wrong or too severe to consider seriously. Imagine saying, "A school would never let kids go without food for months" - in this case, not letting children have enough to eat is an unthinkable situation. The word can be used as a noun or an adjective and usually implies something that's so bad or illogical it doesn't even enter your mind as a possibility.