What is the irony in the line from Macbeth, "hail, most worthy thane"?
Ross has been sent by King Duncan to tell Macbeth, the current Thane of Glamis, that he has been awarded the title Thane of Cawdor as well, for his tremendously loyal and brave services to the crown and country of Scotland. When Ross acquaints him with this news, he says to Macbeth, in part, "hail, most worthy thane." This line is ironic because, as we find out soon enough, Macbeth is not worthy or loyal, ultimately. Poor Duncan has already been deceived by the former Thane of Cawdor, a traitor, and now he believes that he is giving that man's title to another man with a completely different character. Alas, he's deceived there, too, because Macbeth is even more treacherous than his predecessor. While the old Thane of Cawdor turns traitor on the battlefield for all to see, Macbeth plots his treason privately, killing his king -- a king who's also his friend and kinsman and guest -- while he sleeps. Everyone is deceived, at this stage in the play, thinking Macbeth is "worthy" when he is not at all.