The crossword solution for the term 'O Brother, Where Art ___?' is 'THOU', which is a pronoun meaning 'YOU' in Old English. This phrase is taken from William Shakespeare's play 'HAMLET', in which the character HAMLET speaks the line 'O, that this too too solid flesh would melt, Thaw, and resolve itself into a dew!' The full phrase is 'O Brother, Where Art THOU?', which is a reference to the Biblical story of Cain and Abel.