irresponsible

Etymology

From ir- + responsible.

adj

1. (now rare) Not responsible; exempt from legal responsibility, not to be held accountable.
   They left the crown what, in the eye and estimation of law, it had ever been, perfectly irresponsible. 1790, Edmund Burke, Reflections on the Revolution in France, Oxford, published 2009, page 28
2. Lacking a sense of responsibility; performed or acting as though without responsibility; negligent.

noun

1. Someone who is not responsible.