Because it prevents them from breathing, since the ant is blocking off their airway.
Mirrors by Justin Timberlake.
I think informal? Family members commonly use slang :)
Macbeth is feeling invincible in this scene. His mania has risen to a level where he feels he cannot be touched or hurt until Birnam wood comes to his castle, which he believes to be an impossibility. He has taken the witches' prophecies and held them in his mind as if they put him on top of the world. He carelessly yells at his staff and demands that the doctor just simply fix Lady Macbeth. His mood is summed up at the end of the scene when he says "I will not be afraid of death and bane / <span>Till Birnam forest come to Dunsinane". This shows that he will never be afraid of death or being hurt until the forest comes to him.</span>
