What does "unsee" mean? Is it opposite of "see"?
Yes, that's correct! It's actually an informal verb (meaning it is not considered Standard English), but is commonly used in colloquial conversation. When the speaker says "you can't unsee it," she means to say that you can't erase the memory or forget about it. This expression is used to describe things that may be shocking, surprising, or unpleasant. Ex: I just saw some roadkill. I can't unsee it. Ex: The scene was so unpleasant, I can't unsee it.