Source : Webster's Revised Unabridged Dictionary (1913)

Irresponsible \Ir`re*spon"si*ble\, a. [Pref. ir- not +
   responsible: cf. F. irresponsable.]
   1. Nor responsible; not liable or able to answer fro
      consequences; innocent.

   2. Not to be trusted; unreliable.

Source : WordNet®

irresponsible
     adj : showing lack of care for consequences; "behaved like an
           irresponsible idiot"; "hasty and irresponsible action"
           [ant: {responsible}]