童 (tóng): This character means “child” or “youngster.” It represents innocence, inexperience, and vulnerability.
叟 (sǒu): This is a more literary or classical character for an “old man.” It represents the elderly, who might also be seen as vulnerable or deserving of special respect.
无 (wú): A common character meaning “without,” “no,” or “not.” It acts as a negation.
欺 (qī): This character means “to cheat,” “to deceive,” or “to bully.”
How they combine: The phrase literally assembles as “[对待]儿童和老叟, 也无欺骗” ([Treating] children and old men, also without deception). By specifically promising not to cheat the most vulnerable customers, the idiom makes a powerful, all-encompassing statement about a merchant's integrity and commitment to fair prices for everyone.