Meaning:

“年” is short for “年纪” (niánjì; meaning “age”), “下” means “under,” and “男友” means “boyfriend.” Originating from Japanese, this term means “a younger boyfriend.” It recently caught on with a popular Japanese TV series that tell love stories between couples of younger men and older women.

Example:

A: 上次聚会，好像新来的同事对你特别热情。

Shàngcì jùhuì，hǎoxiàng xīnlái de tóngshì duì nǐ tèbié rèqíng。

The new colleague seemed especially enthusiastic towards you during our last gathering.

B: 他看起来年纪很小。

Tā kànqǐlái niánjì hěn xiǎo。

He looks very young.

A: 找个年下男友也不错啊。

Zhǎo ge niánxià nányǒu yě búcuò a。

Finding a younger boyfriend would be okay.