Or HaChaim on Genesis 21:26/אור החיים על בראשית כ״א:כ״ו

1 ויאמר אבימלך…וגם אנכי, Avimelech said: "Also I have not heard, etc." The words: "also I," need clarification; so do the words: "who has done it." All Avimelech needed to say was "I did not know."

2Apparently, Avimelech had been aware that someone had robbed Abraham of his wells but he did not know the identity of the robbers. Abraham therefore told Avimelech that he should have made it his business to find out who the culprits were. To this argument Avimelech replied that Abraham himself was also at fault because he had not informed him previously. He, Avimelech, had waited for Abraham's complaint; he would have taken action as soon as he would have received an official complaint from Abraham. Seeing Abraham had not informed him, he had remained unaware. This is the meaning of: "also I did not hear until this day." He meant that he made no effort to inform himself of the situation and was apprised of it only today when Abraham spoke up.