Source : Webster's Revised Unabridged Dictionary (1913)

Irresponsibility \Ir`re*spon`si*bil"i*ty\, n. [Cf. F.
   irresponsabilit['e].]
   Want of, or freedom from, responsibility or accountability.

Source : WordNet®

irresponsibility
     n : a form of untrustworthiness; the trait of lacking a sense of
         responsibility and not feeling accountable for your
         actions [syn: {irresponsibleness}] [ant: {responsibility},
          {responsibility}]