Fling, sling, etc. mud (at somebody) informal verb phrase
To say bad things about someone to damage their reputation
Ignore him! He flung mud at you simply because he's scared of losing to you.
Kenny is always slinging mud at me just because she is so jealous of my success.
He slings mud at her at every opportunity.
Someone who ruins others' fun and enjoyment
The verbs "fling, sling, etc." should be conjugated according to its tense.
This phrase is believed to allude to the proverb "throw dirt (mud) enough, and some will stick" which means if enough accusations are made against someone, their reputation will suffer, whether they are true or not.
A humorous way of saying that what one has said is just what they acknowledge and may not be entirely accurate
Mike: Have you returned from your vacation on the beach? Your skin seems to get tanned.
Jenny: My skin is always dark. That's my story and I'm sticking to it!