Heart’s Attorney [noun] Tongue.
Simple enough, but the connotation of the phrase is interesting. The tongue is credited with, in this case, controlling what is said in much the same way that an attorney controls what a defendant says.
I wanted to express my distaste at my love’s choice of music, but my heart’s attorney forbade me from doing so, lest our two-hour journey drag on in silence.
Kacirk, Jeffrey. The Word Museum: The Most Remarkable English Ever Forgotten. New York: Simon & Schuster, 2000. 91. Print.