电灯泡: a Chinese slang word for third wheel

When you hear Chinese people say, "Don't be a 电灯泡 (diàn dēng pào)！" Your first reaction may be—"Wait, what? What do you mean by 'don't be a bulb'? Of course I am not a BULB!" However, you will realize that this bulb is not the real bulb; actually it refers to the third wheel.

当你听到中国人说“别做电灯泡”， 你的第一反应可能是“等等，什么？别做电灯泡是什么意思？我当然不是一个电灯泡了”。但是，实际上此电灯泡非彼电灯泡。这里的电灯泡是指那些不合时宜、碍手碍脚的人。

Why does 电灯泡 mean the third wheel? Many people think that it is because the bulb is always too bright for the couple to maintain their romantic atmosphere. Nevertheless, the original and accurate explanation of 电灯泡 is from Cantonese. As a popular Cantonese saying goes, "电灯胆（泡），唔（不）通气". It can be roughly translated as "the bulb is not ventilated (because of its vacuum inside)".

为什么电灯泡是指不合时宜、碍手碍脚的人呢？很多人认为灯泡的光太亮，会影响浪漫气氛。这是其实一个误解。电灯泡的这一意思来源于粤语（广东话）中的一句歇后语“电灯胆（泡），唔（不）通气”，意思是指电灯泡是不能通气的（因为灯泡里面是真空）。

The Chinese word for "not ventilated" （不通气）can also describe people who are unschooled in the way of dealing with other people. Therefore, the derivative meaning of 电灯泡 in this situation is the third wheel, someone who doesn't know how to behave properly when dealing with couples.

在中国，“不通气”是指一个人不懂世情的意思。因此，电灯泡引申为不合时宜的人，也就是那些在情侣约会时不懂回避的人。

For example:

例句：

wǒ bù xǐ huān hé wǒ de qíng lǚ péng yǒu chū qù wán。zuò diàn dēng pào zuì tǎo yàn le！
我不喜欢和我的情侣朋友出去玩。做电灯泡最讨厌了！
I do not like going out with my couple friends. Being the third wheel sucks!

dàn shì lǎo bà lǎo mā làng màn shǐ lǐ de Ted yǐ jīng zuò le bā nián de diàn dēng pào le！tā hǎo xiàng cóng bú jiè yì。
但是老爸老妈浪漫史里的Ted已经做了8年的电灯泡了！他好像从不介意。
But Ted from the TV show How I Met Your Mother has been a third wheel for 8 years. He doesn't seem to mind at all!

hēi，xiǎo nà，xiǎng hé wǒ qù kàn diàn yǐng ma？
A: 嘿，小娜，想和我去看电影吗？
Hey Xiao Na，do you wanna go to the movies with me?

hǎo ā！wǒ yuàn yì！jù shuō fù chóu zhě lián méng fēi cháng……
B: 好啊！我愿意！据说复仇者联盟非常……
Yes! I would love to! I heard The Avengers is really………

hái yǒu wǒ nán péng yǒu？
A: 还有我男朋友？
and my boyfriend?

wǒ jù jué dāng diàn dēng pào！
B:我拒绝当电灯泡！
I refuse to be the third wheel!