incompetent

Lacking the degree of ability and responsibility necessary to do a task successfully.

Near-synonyms: incapable, inable, unable

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.

Opening too early during pregnancy, resulting in miscarriage or premature birth.

Near-synonyms: dysfunctional, nonfunctioning

Not resistant to deformation or flow.

A person who is incompetent.

But besides these incompetents, there was always a train of camp followers, —women who followed the camp, beggars, and criminals, whose number was often greater than the number of fighting soldiers.

"Lies!" said the tallest of the sons of Ivaldi. "I wouldn't trust those fumble-fingered incompetents to shoe a horse."