This is the 13748th most frequent Russian word.
неудавшегося
"Unsuccessful" or "failed."
Here, "неудавшегося" is used to describe the noun "начинания" (undertaking) as something that has not succeeded.
Этот проект был примером неудавшегося начинания.
This project was an example of an unsuccessful undertaking.
In this sentence, "неудавшегося" characterizes the noun "эксперимента" (experiment) to indicate that the experiment did not turn out as planned.
Они обсуждали причину неудавшегося эксперимента.
They were discussing the reason for the failed experiment.
Here, "неудавшегося" modifies the noun "плана" (plan), showing that the plan was ineffective or ended in failure.