A term people use when they want their relationship with their significant other to sound more serious than it actually is. Usually used by insecure or desperate individuals.
I referred to my boyfriend as my future fiance when my landlord asked me who was moving into my place.
A fiancehiss is a fiance who hisses and sometimes growls and acts like a cat. But the rare and very beautiful fiancehiss also purrs when they are loved by their cat.