What is the meaning of "may his forehead be covered with dust" in the Prophet's (peace be upon him) saying to his Companions when admonishing them: "What is wrong with him, may his forehead be covered with dust"?

Summary of the Fatwa:

Al-Bukhari narrated from Anas ibn Malik that the Prophet (peace be upon him) was not a reviler, nor an obscene speaker, nor a curser. When he was displeased, he would say: "What is wrong with him, may his forehead be dusty!" Al-Hafiz Ibn Hajar explained that Al-Khattabi considered two meanings for the phrase: The first is that the meaning is "he fell on his face and dirt touched his forehead," and this is the more probable meaning. The second is that it is a prayer for him to worship, as if he prays and his forehead becomes dusty. Ibn Hajar considered this latter meaning unlikely. Al-Dawudi mentioned that this phrase was common on the tongues of Arabs and its literal meaning was not intended, similar to "may his nose be rubbed in the dust" and "may your right hand be dusty."