A person whose mouth might as well be an anus because all that comes out of it is bullshit.
The joke is that since
shit comes out of an ass, and name-calling and spreading slander or lies are called talking
shit, that means their mouth is an anus since
shit-quality words are all that comes out of it.
Bob: Climate change is a hoax.
Alice: What? No it isn't! Haven't you noticed the jet stream weakening?
Bob: Also, vaccines don't work.
Alice: Wow, you're an assmouth. Try saying something LOGICAL next time.
Bob: Hmmphh! Fine!