1 Samuel 18:19
But it came to pass at the time when Merab Saul’s daughter should have been given to David, that she was given unto Adriel the Meholathite to wife. But it came to pass at the time when Merab Saul’s daughter should have been given to David, that she was given unto Adriel the Meholathite to wife.
Saul had promised his eldest daughter Merab to David as his wife. When the appointed time came for her to be given to David, she was given instead to Adriel the Meholathite, so Saul broke his promise. That failure to keep his word revealed his insincerity in offering the marriage as a reward for David's valor.