The characters combine in a very literal and parallel structure: “To be generous/thick with this one (此), and to be meager/thin with that one (彼).” This beautiful symmetry makes the idiom's meaning of unfair comparison immediately clear.
The concept of 厚此薄彼 is deeply rooted in the Chinese emphasis on fairness (公平, gōngpíng) and social harmony (和谐, héxié). To accuse someone of 厚此薄彼 is to accuse them of disrupting this harmony by creating jealousy and resentment through unjust actions. A useful comparison for Western learners is the concept of “playing favorites.” However, 厚此薄彼 often carries a heavier weight. While “playing favorites” can sometimes be seen as a minor, understandable human flaw (e.g., a teacher liking a cheerful student), being accused of 厚此薄彼 in a Chinese context can be a serious moral criticism, implying a failure of one's duty or responsibility. For example, in a family, parents are expected to provide for their children as equally as possible. In a company, a manager is expected to judge employees based on merit. When this expectation is violated, it's not just “preference”—it's a breach of a social contract. This is particularly sensitive in a culture where relationships (关系, guānxi) are so important. While it's normal to treat “insiders” better than “outsiders” (亲疏有别, qīn shū yǒu bié), excessive or blatant favoritism within the same group is seen as a serious failing.
厚此薄彼 is a formal idiom but is widely understood and used in both spoken and written Chinese, especially when criticizing unfairness.
The connotation is almost universally negative. It is an accusation or a piece of advice against being unfair. You would not use it to describe your own actions unless you were being self-critical.