incompetent
n.n. not having the skills or ability to do a job or task correctly. You use this to describe someone who fails because they don't know what they are doing.
n. lacking the necessary ability, knowledge, or skill to perform a task or function effectively. Often carries a strong pejorative tone when applied to professional conduct.
The incompetent manager lost the company a lot of money.
The board of directors fired the CEO after a series of incompetent decisions led to a total market collapse.
While the junior staff were eager to help, their incompetent handling of the delicate equipment resulted in several costly delays that the project could ill afford.
Borrowed from French incompétent, from Late Latin incompetentem, from Latin incompetēns, equivalent to in- + competent.
Typically used as an attributive adjective before a noun or as a predicative adjective after a linking verb like 'be' or 'seem'.