Unsuccessful means not achieving what was hoped. An unsuccessful attempt, unsuccessful applicants, largely unsuccessful.
It is the polite formula for failure and it is used most in letters nobody wants to receive — if you have been unsuccessful on this occasion. It judges the outcome and not the effort, which is why it stings less than failed. Note that the un- attaches to successful and not to success.
1adjectiveunsuccessfulNot achieving what was hoped.
“After three unsuccessful attempts, he finally passed his driving test.”
unsuccessful is a quality of failure.
From un- + successful.
Etymology from Wiktionary, CC BY-SA