Charles Ellicott Commentary 2 Samuel 18:20

"And Joab said unto him, Thou shalt not be the bearer of tidings this day, but thou shalt bear tidings another day; but this day thou shalt bear no tidings, because the king`s son is dead." — 2 Samuel 18:20 (ASV)

You shall bear no tidings. —Ahimaaz appears to have been in favour both with David (Compare to 2 Samuel 18:27) and with Joab. Joab, therefore, well knowing how painful the news of Absalom’s death would be to David, refused to let Ahimaaz bear it. The word is used, with rare exceptions, to mean good tidings.