As the ancients said: to defame in one's belly and slander in one's heart (i.e. silent disapproval).
Tips
usage
诽 rarely stands alone in modern Chinese, almost always part of compounds: 诽谤 (to defame/slander, in legal language), 腹诽 (silent inner criticism), 怨诽 (to gripe and slander).
memory
Note 谤 is open accusation; 诽 is muttered behind the back. The 言 'speech' radical plus 非 'wrong' hints at 'words against', speech directed at someone's faults.
Speech radical on the left, left-side form of 言, marks 诽 as a speech act, in this case slanderous talk. The same radical drives 说 (speak), 谎 (lie), 谤 (defame), 讥 (mock).
非 supplies the sound (fēi to fěi) through a tone shift, and adds heavy semantic loading: slander is speech 讠 about what is not 非 true. The same 非 phonetic drives 排, 啡, and 悲.
