From kata (against) + laleō (to speak). Katalaleō means to speak against, slander, defame, speak evil of. The noun katalalia (G2636) is slander, evil speech. Used in Peter and James for speech that tears down the community.
The prohibition of katalaleō — slander and evil speech — is one of the most consistently applied social ethics of the NT epistles. 1 Peter 2:1 lists it among the first vices to be laid aside: 'rid yourselves of all malice and all deceit, hypocrisy, envy, and slander (katalalias).' 1 Peter 2:12 calls believers to live such honorable lives that those who accuse them of doing wrong (katalalōsin) will ultimately glorify God by observing their good deeds. James 4:11 delivers the deepest theological indictment: 'Anyone who speaks against a brother or sister or judges them speaks against the law and judges it... there is only one Lawgiver and Judge.' To slander a fellow believer is to set yourself above God's law. The antidote is speech that builds up (oikodomeō), that gives grace to those who hear (Eph. 4:29) — words that carry the construction project of God's kingdom rather than demolishing it.