noun
dirty joke; blue joke
A joke or story with sexual or scatological content. Casual register.
彼は飲み会でいつも下ネタばかり言う。
He always tells dirty jokes at drinking parties.
noun
indecent topic; bawdy subject; sex talk
A conversation topic that is sexually suggestive or crude. Broader than just jokes; can refer to any lewd discussion.
職場で下ネタは避けたほうがいい。
It's better to avoid indecent topics at work.
Compound of 下 (しも, 'lower, vulgar') and ネタ (slang for 'material, topic', from 種 (たね) reversed). The exact derivation of ネタ is uncertain, but it is widely used in modern Japanese for 'story material' or 'joke premise'.