“Sir,” said I, “if I tell you my story, I must commit a friend’s life to your discretion. Pass me your word it shall be sacred; and for what touches myself, I will ask no better guarantee than just your face.” He passed me his word very seriously. “But,” said he, “these are rather alarming prolocutions; and if there are in your story any little jostles to the law, I would beg you to bear in mind that I am a lawyer, and pass lightly.”
Source: wiktionary