What is Mental Reservation?

Legal Definition
The doctrine of mental reservation, or of mental equivocation, was a special branch of casuistry (case-based reasoning) developed in the late Middle Ages and the Renaissance, and most often associated with the Jesuits.
-- Wikipedia
Legal Definition
A silent exception to the general words of a promise or agreement not expressed, on account of a general understanding on the subject. But the word has been applied to an exception existing in the mind of the one party only, and has been degraded to signify a dishonest excuse for evading or infringing a promise. Wharton.
-- Black's Law Dictionary