said, done, or characterized by a lack of a sense of responsibility

His refusal to work shows him to be completely irresponsible.

not capable of or qualified for responsibility, as due to age, circumstances, or a mental deficiency.

not responsible, answerable, or accountable to higher authority

irresponsible as a monarch.

an irresponsible person.

adjective

accountable , careful , liable , reliable , responsible , trustworthy

adjective

capricious , carefree , devil-may-care * , feckless , fickle , flighty , fly-by-night * , giddy , harebrained , ill-considered , immature , immoral , incautious , lax , loose * , no-account , rash , scatterbrained * , shiftless , thoughtless , unaccountable , unanswerable , uncareful , undependable , unpredictable , unreliable , unstable , untrustworthy , wild , heedless , inattentive , reckless , unconcerned , unmindful , unthinking , arbitrary , careless , foolish , harum-scarum , impetuous , impractical , impulsive , irrational , scatterbrained , visionary