incompetent

Definition from Nolo’s Plain-English Law Dictionary

1) Unable to manage one's affairs due to mental incapacity or sometimes physical disability. Incompetence can be the basis for the appointment of a guardian or conservator to handle the incapacitated person's affairs. 2) In criminal law, unable to understand the nature of a trial, and therefore not qualified to stand trial or testify. 3) A general reference to evidence that is not admissible at trial.

Definition provided by Nolo’s Plain-English Law Dictionary.