indefinably       
adverb (also undefinably) uk/ˌɪn.dɪˈfaɪ.nə.bli/ us/ˌɪn.dɪˈfaɪ.nə.bli/
 难下定义地;难以确切描述(或解释)地
in a way that is impossible to clearly describe or explain
  The two versions are indefinably different. 这两个版本有着难以界定的不同。 
 He seemed indefinably weary. 他似乎有种说不出的疲惫。 
 See: indefinable
 - More examples
 - Something in the music is indefinably familiar.
 - There is something about him that is indefinably larger than life.
 - Her accent sounded indefinably foreign.