A Japanese expression often used to refer to an extremely small strip of land or narrow room. The implication is that a cat could barely squeeze itself into the space (the forehead being the widest part of any fit cat.)
Many Japanese apaato, especially in big cities, are longer than they are wide, often to the point where two people can barely pass in kitchenettes or other bottlenecks. This expression also commonly refers to back gardens or walkways.
Uchi no daitokoro wa neko no hitai da.
The kitchen in my house is (as narrow as) a cat's forehead.
The adjective semai is often used to the same purpose as this expression, but there is one major difference. When referring to one's house, one often uses semai (implying "Sorry our house is so small, it must be uncomfortable for you") to express humility towards one's guests, even if the house is quite large. "Neko no hitai" is more emphatic than "semai", and normally used as an honest complaint rather than a humilifier (although both are certainly possible).
This is also a very bad expression to mistake with neko no shitai, a cat's corpse.