What is Index Animi Sermo?

Legal Definition
Language is the exponent of the intention. The language of a statute or instrument is the best guide to the intention. Broom, Max. 622.
-- Black's Law Dictionary
Legal Definition
Speech is the index of intent. See 41 Neb. 631, 25 L. R. A. 564, 59 N. W. 935.
-- Ballentine's Law Dictionary