It is always the best policy to speak the truth, unless, of course, you are an exceptionally good liar.

The line is a characteristically Jeromian epigram: it endorses the moral commonplace that honesty is prudent (“the best policy”), then punctures it with a comic exception that exposes how often “policy” rather than principle motivates truth-telling. By granting an out to the “exceptionally good liar,” the quip satirizes social life in which deception can be rewarded if performed skillfully, and it hints at the Victorian/Edwardian tension between public respectability and private maneuvering. The humor depends on reversing ethical advice into a practical calculus: truth is safest not because it is right, but because most people are not competent at sustaining lies.