M'lud is how a lawyer in a British courtroom used to address the judge. It is my lord as it sounds when said fast and a bit grand; people write it that way to catch the sound. It belongs to British law drama and old-fashioned comedy, not to American courts (there the judge is Your Honor) and not to Taiwan. It feels elderly and upper-class, and writers use it when they want the judge to seem a little out of touch. There is a well-worn story that a barrister once had to explain the Beatles to a judge as a popular beat combo, m'lud. Whether or not that exact moment happened, the joke stuck because it paints the judge as grand and behind the times. You only need this word to understand British legal scenes.

法官大人（英國法庭對法官的稱呼，從 my lord 變音而來；台灣法庭說「庭上」）