I don't know how it is written, but it was subtitled as "she's weird". For context: It was thought by one character after seeing another essentially tell others to go away so she could sleep in class.
I could only find definitions for "hen" and "atsu". "Hen" could mean "strange", and "atsu" could be a shortening of "atsukamashi", which means "impudent"/"shameless".
With "nai" in the middle, the closest I can come up with is "not just strange, but shameless", but I'm super iffy on that.
Any help will be greatly appreciated!