无形无影 Invisible and untraceable
Explanation
形容事物没有形体和影子，看不见摸不着，也形容事物消失得无影无踪。
Describes something that has no form or shadow, cannot be seen or touched, and also describes something that has disappeared without a trace.
Origin Story
传说在古老的秦岭深处，住着一个神秘的女子。她轻盈如燕，身法飘忽，来无影去无踪，宛如山间的精灵。村民们都说她是山神派来的使者，守护着这片古老的山林。一日，山林中出现了一群盗猎者，他们肆意砍伐树木，猎杀珍禽异兽。山神震怒，派女子前去阻止。女子身姿轻盈，如风般穿梭于树林之间，盗猎者们只觉得眼前一花，便被她用巧妙的身法制伏，手中的猎物和工具也纷纷落地。待盗猎者们回过神来，女子已无形无影，只留下山林中回荡的轻风低吟。此后，再也没有人敢在山林中为非作歹，古老的山林恢复了往日的宁静祥和。
Legend has it that in the depths of the ancient Qinling Mountains, there lived a mysterious woman. She was as light as a swallow, her movements were erratic, and she came and went without a trace, like a spirit in the mountains. Villagers said she was a messenger sent by the mountain god to protect this ancient forest. One day, a group of poachers appeared in the forest, wantonly felling trees and hunting rare birds and beasts. The mountain god was furious and sent the woman to stop them. The woman's body was light and agile, and she shuttled through the forest like the wind. The poachers only saw a flash of light before they were subdued by her skillful movements, and their prey and tools fell to the ground. When the poachers came to their senses, the woman had disappeared without a trace, leaving only the gentle whisper of the wind echoing in the forest. After that, no one dared to do evil in the forest again, and the ancient forest returned to its former peace and harmony.
Usage
常用来形容事物或行动难以察觉，神秘莫测。
Often used to describe things or actions that are difficult to detect, mysterious, and unpredictable.
Examples
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他做事总是神出鬼没，无形无影。
tā zuòshì zǒngshì shénchūguǐmò, wú xíng wú yǐng
His actions are always elusive and imperceptible.
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敌人的行动非常诡秘，无形无影，令人防不胜防。
dírén de xíngdòng fēicháng guǐmì, wú xíng wú yǐng, lìng rén fáng bù shèng fáng
The enemy's movements are extremely secretive and invisible, making it difficult to guard against.