To hate someone or something with a passion, to the point of feeling sick when it or they near...
To show invollentary physical signs(shuddering, cringing, face or body twitching, eyes-rolling) of hate towards an Item or Person when it nears.
That person that makes you cringe at first sight... you're mentally allergic to them.
That country music singer's voice that sickens you at first sound... You're mentally allergic to it.
That certain food that the sight/smell/taste of makes you wanna puke... you may not be physically allergic to it, but you are mentally allergic to it!!!
Cool guy: standin' chillin.
Village idiot: "hey dude, What's up?"
Cool guy: Shuddering, body shaking, green in the face, "get away from me!"
Village idiot: "what's wrong?"
Cool guy: "I'm mentally allergic to you!"