Combining them, the literal meaning is “in the heart (心) one holds (怀) that which cannot be (叵) measured (测).” This beautifully illustrates the idea of a person whose true, wicked intentions are so deep and hidden that they are completely unfathomable to an outsider.
In Chinese culture, social harmony and trust are highly valued. Sincerity (诚, chéng) and trustworthiness (信, xìn) are foundational virtues. The idiom 心怀叵测 strikes at the heart of these values, describing the ultimate social betrayal: someone who appears normal or even friendly on the surface but is secretly plotting harm. This is different from the Western concept of being “two-faced.” While “two-faced” often implies hypocrisy or saying different things to different people, 心怀叵测 goes deeper. It's not just about inconsistency; it's about a premeditated, malicious plan. The core of the meaning is the “unfathomable” (叵测) nature of the person's evil. They are a “wolf in sheep's clothing,” but one whose strategy is so cleverly concealed that their danger is not immediately apparent. This term is often used in stories and discussions about political intrigue, business backstabbing, or deep personal betrayal.
心怀叵测 is a formal, literary idiom with a very strong negative connotation. It is not used in casual or lighthearted conversation.