Be it a misconception, a fantasy, or a thought too devious to share, something that is inter-mental exists only in your head and quite often should be kept that way.
Can also be used to describe a similar thought shared by two individuals who suddenly realize they are thinking the same thing without even saying it out loud.
"Sick, dude. Next time you have a thought like that, keep it inter-mental."
"That's ok, baby. You don't have to tell me what you want. I got that inter-mental."
When that big girl in the ghetto fabulous outfit with her bra showing through the key hole back in her shirt and the frilly purple mini-skirt that doesn't even cover her butt walked by us, we were both overcome with inter-mental amusement and shared a laugh without saying a word.