adjective / noun
incompetent
in-KAHM-puh-tunt/ɪnˈkɑːmpɪtənt/
Lacking the capacity required for an action.
The incompetent assembler struggles with a stool whose legs splay unevenly.

Etymology
Competere's sense of agreeing or coinciding underlies the idea of being suitable or meeting requirements. With the negative prefix, the word first meant insufficient and then specifically lacking the ability or qualifications needed.
In context
If they eat porridge mixed with sand, they will surely curse the court for being corrupt and call your majesty an incompetent ruler.
More examples
- He is incompetent at even basic bookkeeping.
- The judge finds him incompetent to stand trial.
- We cannot trust an incompetent to manage the budget.
Forms
incompetent · incompetents
incompetence noun — Lack of required skill or legal capacity.
incompetently adverb — Without the skill needed to perform adequately.