a.

1. 无法解释的，不可理解的；莫名其妙的
   I often suffer from unaccountable headaches.

   我常莫名其妙地头疼。

2. 无责任的[（+for/to）]
   An insane person is unaccountable for his actions.

   精神病患者对其行为不负任何责任。

adj.

1. unable to be explained.
2. not responsible for or required to justify consequences.