"Bad mouthing" refers to speaking negatively about someone or something, usually in an unfair or malicious way. It involves making unkind remarks or spreading rumors to harm a person or their reputation. Full definition
There are certain financial «gurus» out there who bad mouth whole life, despite the many benefits that cash value life insurance offers. (termlife2go.com)
Never bad mouth a former employer or colleagues; it doesn't reflect positively on you. (michellecookkaufmann.com)