Boyd’s Bible Dictionary:
To bind under a curse (Josh. 6:26; 1 Sam. 14:24). To require a declaration of truth at the peril of God’s displeasure (Matt. 26:63).
Concise Bible Dictionary:
A solemn charge by one in authority to another to speak the truth under the obligation of an oath, as when Ahab adjured Micaiah (1 Kings 22:16), and when the high priest adjured our Lord (Matt. 26:63). Also binding a person under a curse to do or not to do the thing enjoined, as in Joshua 6:26; 1 Samuel 14:24 (compare also Mark 5:7; Acts 19:13). Also a solemn charge, as in 1 Thessalonians 5:27, in the margin.