impossible

(noun) something that cannot be done. Example: "His assignment verged on the impossible."

(adj) not capable of occurring or being accomplished or dealt with. Synonyms: impossible (vs. possible). Examples: "An impossible dream." "An impossible situation."

(adj) totally unlikely. Synonyms: inconceivable, out of the question, unimaginable.

(adj) used of persons or their behavior. Synonyms: insufferable, unacceptable, unsufferable. Examples: "Impossible behavior." "Insufferable insolence."