BER lists two hypotheses: * From a transposed figurative sense of ни- (ni-, “non-”) + един (edin, “one”), cognate with Serbo-Croatian нијѐдан (“no one”). * A relict of Proto-Balto-Slavic *náiˀdas (“animosity”) + -ен (-en), akin to Latvian nâids (“hatred, discord”), Lithuanian paniedètas (“despicable”). Favoured by Iljinsky.