incompetent

Short definition from Wiktionary

Adjective

1. 1
   Lacking the degree of ability and responsibility necessary to do a task successfully.
2. 2
   Lacking the degree of ability and responsibility necessary to do a task successfully: Unable to make rational decisions; insane or otherwise cognitively impaired.
3. 3
   Lacking the degree of ability and responsibility necessary to do a task successfully: Opening too early during pregnancy, resulting in miscarriage or premature birth.
4. 4
   Lacking the degree of ability and responsibility necessary to do a task successfully: Not resistant to deformation or flow.

Noun

1. 1
   A person who is incompetent.