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English Translation

lit.: to quench one's thirst by thinking of plums
fig.: to console oneself with false hopes, to feed on fancies

Breaking it down

1. 望: to hope, expect
2. 梅: plum
3. 止: to stop
4. 渴: thirst

Source: Google Image Search

Story in Chinese

Classical Chinese

南朝宋·刘义庆《世说新语·假谲》：“魏武行役失汲道，军皆渴，乃令曰：‘前有大梅林，饶子，甘酸可以解渴。’士卒闻之，口皆出水，乘此得及前源。”

Modern Chinese

魏武帝（曹操）行军途中，找不到水源，士兵们都非常口渴，于是他传令道：" 前边有一片梅子林，结了很多果子，梅子酸甜可以解渴。"
士兵听了后，嘴里的口水都流了出来，曹操利用这个办法促使部队尽快赶到了前方，找到了水源。

Source: baike.baidu.com

Story in English

Emperor Wu of Wei, Cao Cao was marching with his troops, but they couldn't find any water. The soldiers were very thirsty, and so Cao Cao said to them: "In front of us there is a forest of plum trees, which bears a lot of fruits. Those sweet and sour plums can quench your thirst."
When the soldiers heart this, their mouths got watery, and Cao Cao used this method to move the troops forward quickly, so that they could find water.

Translation by Celine Li

Usage

他没钱买别墅，只好天天看着房屋广告望梅止渴了。
一路上将军一直用望梅止渴的方法刺激着队伍走出了沙漠。