Definition from Wiktionary, the free dictionary
- Unskilled, lacking normally expected degree of ability.
- Having an incompetent lawyer may be grounds for a retrial, but the lawyer in question probably doesn't know that.
- Unable to make rational decisions, insane or otherwise cognitively impaired.
- The charged was judged incompetent to stand trial, at least until his medication started working.
- See also Wikisaurus:bad
unskilled, lacking normally expected degree of ability
unable to make rational decisions, insane or otherwise cognitively impaired
Usage notes 
The form incompetent of, as in "I am incompetent of doing anything!" is heard (US). It is incorrect, the intended word is incapable.
incompetent (plural incompetents)
- A person who is incompetent.