To sing the "Empty City Strategem"
Refers to using some kind of method to conceal one's weakness from the enemy. Also can indicate that something or one's stomach is empty, or that a company's employees are mostly out of the office (for group travel, etc)
Neutral
最近我遇到的唯一个难题是我的邮箱常常唱空城计 | Zuìjìn wǒ yù dào de wéi yīgè nántí shì wǒ de yóuxiāng chángcháng chàngkōngchéngjì
My only problem as of late is that my mail is empty
[Source]
天渐渐黑了下来，我的肚子早就唱空城计了 | Tiān jiànjiàn hēi le xiàlái, wǒ de dùzi zǎo jiù chàng kōngchéngjì le
It got dark out slowly. My stomach's been rumbling for ages
[Source]
None
None
A 古琴 (gǔ qín) is an ancient instrument that is similar to the modern-day zither (古筝, gǔ zhēng). It's a string instrument like harp, except it rests horizontally atop a wooden box. (Please just Google it.)
《三国志》(Sān Guó Zhì) by 陈寿 (Chén Shòu). The historical text covers the events of the end of the Eastern Hàn Dynasty (东汉, Dōng Hàn, 25 - 220) and the Three Kingdoms period (220 - 280)
During the Three Kingdoms period (220 - 280), the state of 魏 (Wèi) ordered its general 司马懿 (Sī Mǎ Yì) to lead an assault against the invading 蜀 (Shǔ) kingdom army. When 司马懿 won a decisive victory, he and his troops pressed onward to attack the now-exposed 蜀 city of 西城 (Xī Chéng).
Word of 司马懿's imminent attack quickly made its way back to the city and 诸葛亮 (Zhū Gě Liàng), the 蜀 military strategist stationed inside it. With most of the 蜀 army deployed elsewhere, he realized that unless he devised a way to stave off the attack, 西城 would almost certainly fall to 魏 hands. As such, he ordered the city to throw open its main gates, and he personally climbed atop the gate tower and started calmly playing the 古琴 (gǔ qín).
司马懿 and his troops arrived soon after, and the general was floored at the sight of the open gates and the obviously unperturbed 诸葛亮 strumming his instrument atop the city walls. He knew that the strategist was a genius, so he couldn't help but fear that this nonchalance was a sign of an imminent ambush. He promptly ordered a full retreat.