1. impossible to explain or understand
2. not required to justify actions or decisions to others

Examples

• For some unaccountable reason, the computer stopped working.
• Her sudden change in behavior was completely unaccountable.
• The CEO seemed unaccountable to the board of directors.
• It’s unaccountable how he managed to disappear so quickly.
• The agency operated with unaccountable power for years.
• His unaccountable absence worried his family.
• The government official remained unaccountable for his decisions.