Moral of this idiom: Not all offensive words have a bad heart, just as not all smiles come from a kind heart. There are two kinds of bad guys, one is an obvious wolf, and the other is a wolf in sheep's clothing. The former, because people are easy to identify and learn skills from personal experience and hearsay, is naturally wary of it.
But the latter, under its kind face like a sheep, people are gradually completely unprepared and will be attacked like water inadvertently. Idioms tell people to get along with others, make friends, and don't quickly define each other's characteristics based on the first impression. Before deciding how to continue your relationship, you should spend enough time paying attention to each other's behavior.
Idiom story:
At the end of the Eastern Han Dynasty, Jingzhou became a battleground for military strategists because of its very important geographical position. In AD 2 17, Lu Su died of illness, and the honeymoon period of Sun and Anti-Cao ended. At that time, Guan Yu was guarding Jingzhou, and Sun Quan had long wanted to seize Jingzhou, but the time was not yet ripe. Soon after, Guan Yu sent troops to attack Fancheng controlled by Cao Cao, fearing future troubles, leaving heavy troops stationed in public security and Nanjun to defend Jingzhou.
Monroe, a general under Sun Quan, thought it was time to capture Jingzhou, but because of illness, he suggested that Sun Quan send Lu Xun, a young general who was unknown at that time, to take his place and be stationed in Lukou. Lu Xun was not obvious when he took office, so he decided to make up with Guan Yu and really prepare for the war. He wrote a letter to Guan Yu, in which he boasted about Guan Yu, saying that Guan Yu was as successful as Jin Wengong and Han Xin.
Baidu Encyclopedia-Hide the Knife in the Smile