na-adjective
incomprehensible; impossible to understand
Used when something is beyond understanding or cannot be deciphered, often with a nuance of frustration or bewilderment.
彼の行動は理解不能だ。
His behavior is incomprehensible.
この暗号は理解不能で、誰も解読できなかった。
This code was impossible to decipher, and no one could crack it.
不可解 also means 'incomprehensible' but often implies something mysterious or puzzling, while 理解不能 is a more direct statement that understanding is impossible.
Compound of 理解 (understanding) and 不能 (impossible, incapable).