Unfathomable means that something or some one is unbelievable you just can not believe it. also. unfathomable; adjective 1 incapable of being fully explored or understood. 2 impossible to measure the depth or extent of. Unfathomable means that something or some one is unbelievable you just can not believe it. also unfathomable; adjective 1 incapable of being fully explored or understood. 2 impossible to measure the depth or extent of.