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1.
when the butt is so flat that it connects straight into the thigh;

or when the butt and thigh are so big that you cannot tell which is the butt or the thigh
she has a butt-thigh!!
by Juju Huckabee September 19, 2006
 
2.
the absence of a butt, when one's butt merely looks as if it were part of one's thigh. Comes from the ancient Greek goddess, Rebecca.
Damn, that girl has no ass- she has a butt thigh!
by Lindsay August 28, 2004