“嘴” means “mouth,” and “炮” means “guns.” This term, first used by netizens in Taiwan, refers to people who like to brag about things they often cannot accomplish. The term has caught on among netizens on the Chinese mainland as well. The image is vivid as if one could use his mouth as a machine gun.
Xiǎomíng shuō yào hé nǐ bǐsài tóu sānfènqiú，tā shuō tā kǔliàn le yīgè yuè，xiànzài bǎifā bǎizhòng。
Xiaoming said he wants to challenge you to a game of shooting three-pointers. He said he practiced for a month and would not miss a single shot.
Tā jiùshì zuǐpào，wǒ cái bú pà tā ne。
He’s just bragging. I am not afraid of him at all.