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.