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.
To say the same thing repeatedly
Ok, I get it! You sound like a broken record, seriously!