Meaning:

“九十度” means “a 90-degree angle,” and “青年” means “a young person.” This term coined by Chinese netizens refers to those introverted young people who feel uncomfortable in a crowded social function, and prefer to find shelter in a corner of the room (where the conjoining walls form a 90-degree angle) and become almost invisible to others. They choose to stand back rather than getting involved, as they have no desire to be the center of attention. This is similar to the English term “wallflower.”

Example:

A: 明天小李过生日，请同事去他家参加派对，你去不去？

Míngtiān xiǎolǐ guò shēngrì，qǐng tóngshì qù tājiā cānjiā pàiduì，nǐ qùbuqù？

Tomorrow is Xiao Li’s birthday and he has invited his colleagues to attend a party at his home. Will you go?

B: 算了，你知道我是个九十度青年。

Suàn le，nǐ zhīdào wǒ shì gè jiǔshídù qīngnián。

I’ll pass. You know I’m one of those wallflowers who don’t really enjoy socializing.