The verb bewail means to lament or express great sorrow. When your big brother or sister starts kindergarten, you may bewail the fact that they can't play with you all day anymore but you'll be excited when you get to go to school also!
The verb bewail is from the Old Norse word væla, meaning to lament. The prefix be- is added when you want to make something stronger or more intense. So bewail means to greatly lament and when compared to bemoan, a word with similar meaning, bewailing would be louder and more intense. You can bewail the death of a friend, but if you bewail a minor irritation like a broken nail, people will accuse you of being overly dramatic.