胡说(hú shuō) - to talk nonsense; nonsense, rubbish in Mandarin

胡说 (hú shuō) is to talk nonsense: to say things with no basis, whether from ignorance, invention or drink. It is a verb, so 他在胡说 is he is talking rubbish, and it doubles as a noun in 这是胡说, this is nonsense.

On its own, barked out, it is an interjection meaning Rubbish! or Nonsense!. The longer 胡说八道 is the fixed four-character version and hits harder still. Both are blunt: you are telling someone directly that what they just said is worthless.

The trap is that it accuses, and English speakers reaching for a light that is silly often land here by accident. 胡说 is not playful. It is also not quite lying: 撒谎 means to tell a deliberate untruth, while 胡说 covers rambling nonsense as well, whether or not the speaker knows better. If you only want to say a claim is mistaken, 不对 or 你搞错了 will not start an argument.