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!