Almost always 诋毁 ('to slander / smear') — to deliberately attack someone's reputation with false or malicious words. Compare 诽谤 (to defame), which is closer to the legal sense.
Speech radical on the left, side-form of 言. It marks an act of words — here malicious, defaming speech. Same family as 谤 to defame, 诽 to slander, 讥 to mock.
