Regrettable, Unsatisfactory
noun, な adjective
If you leave behind something and it makes you feel remorse it's a really regrettable situation. You should probably go back for whatever it was so you won't lead an unsatisfactory life.
いかん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
裁判は誠に遺憾な結果に終わりました。
The trial ended with an absolutely unsatisfactory result.