WordNet sense 1 (not clearly understood or expressed):

opaque

WordNet sense 2 (impossible to understand):

impenetrable

WordNet sense 3 (impossible to come to understand):

unfathomable

WordNet sense 4 (not clear to the understanding):

enigmatic

WordNet sense 5 (marked by difficulty of style or expression):

obscure

WordNet sense 6 (difficult to understand):

incomprehensible