a fly in the ointment 點解解作掃興

「ointment 」這個字，是指用於皮膚的珍貴物品，可以是化妝或醫藥用；高黏性或半固態。如果上面竟然浮著一隻蒼蠅，那這珍貴而難得的油就不能用了。

在今天的世界，甚麼都可以買，現代人可能不明白以前的社會「ointment 」的難得。你想通這「難得」的一點，應該就很容易明白「ointment 」上的蒼蠅給人的掃興的感覺了。

A/the fly in the ointment

Meaning: something/someone that spoils a perfect or very satisfactory situation, plan or state of affairs

Usage:
Jack: "Apparently they don't like poor Lennie."
Jone: "Well, neither do I."
Jack: "He's the only fly in the ointment around here, as far as I'm concerned."

Chinese Translation: 美中不足