A person who or thing which has or claims authority to decide upon a matter without reference to others. Chiefly in predicative use.
Late 16th century; earliest use found in George Whetstone (d. 1587), writer. From judge + and + jury.
judge and jury/ˌdʒʌdʒ ən(d) ˈdʒʊəri/
with object To give consideration to or pass judgement upon as a judge and jury, or in the manner of a judge and jury; to try to pass sentence upon.
Late 19th century; earliest use found in Thomas Hardy (1840–1928), novelist and poet. From judge and jury.