A nomological dangler refers to the occurrence of something (in this case a sensation) that does not fit into the system of established laws. Smart believes that it is absurd that everything can be explained by the laws of physics except consciousness. He identifies consciousness with the broad term "sensations".
A nomological dangler is a phenomenon that doesn't conform to the existing set of rules. Smart argues that it's unreasonable that all phenomena can be accounted for by physical laws, but not consciousness, which he identifies as encompassing sensations.