The term 'I DON'T BELIEVE YOU!' can be expressed in a variety of ways, as evidenced by the crossword solutions given. The four letter solution, 'BULL, ' is an expression used to describe something that is not true or is false. The thirteen letter solution, 'HORSEFEATHERS, ' is an old-fashioned phrase used to express disbelief or disagreement. The four letter solution, 'LIAR, ' is a noun used to describe someone who tells lies or untruths. The five letter solution, 'MYEYE, ' is an expression used to express disbelief or doubt. Lastly, the six letter solution, 'CANTBE, ' is an expression used to express disbelief or doubt. All of these solutions are used to express disbelief or disagreement with the statement 'I DON'T BELIEVE YOU!'