Matthew 6:3 - Coke's Commentary on the Holy Bible

Let not thy left hand know— This is a kind of proverbial expression, which may be explained to this effect: "Let no one, no, not even your most intimate acquaintance, know what you do; be ignorant of it yourselves, or forget it immediately as far as possible." It is said that the poors' chest stood on the right hand as they entered the synagogues; to which circumstance some suppose the words to all