In the late Eastern Han Dynasty, after the great Wu general Zhou Yu died, Liu Bei was in Jingzhou discussing plans to enter Shu with Zhuge Liang. When news reached Cao Cao that Liu Bei was recruiting troops and allying with Wu for a northern campaign, he gathered his advisors to discuss a southern expedition. Cao Cao worried that if he marched south, the Xiliang governor Ma Teng might attack his base at Xuchang. Advisor Xun You proposed, "We can issue an imperial decree appointing Ma Teng as General of the Southern Expedition, ordering him to attack Sun Quan, then lure his forces to Xuchang and eliminate them." Cao Cao agreed wholeheartedly, so he sent an envoy to Xiliang with the decree summoning Ma Teng to the capital.
When Ma Teng received the imperial summons, he gathered his sons and nephews to debate whether to go. His eldest son Ma Chao urged acceptance, but his nephew Ma Dai objected, "Cao Cao harbors treacherous intentions, Uncle. If you go, I fear he will harm you." Dismissing the warning, Ma Teng arrogantly took his son Ma Xiu and a few Liangzhou troops to the capital Xuchang, where Cao Cao ultimately had him killed in a trap.
Later, the idiom "harboring unfathomable designs" came to describe concealing malicious intent or an unpredictable scheme.
Source: *Romance of the Three Kingdoms*
Meaning of the Idiom: Later, the Chinese idiom "心怀叵测" came to describe concealing malicious intent or an unpredictable scheme.