Ruiz uses “impeccable” to mean only using words that express or manifest love and positivity (which is the opposite of how society usually trains people to talk). Agreeing to be “impeccable with your word,” therefore, means agreeing not to say anything negative, divisive, or judgmental. A person who says judgmental things about others invites judgment back, so talking negatively about others also manifests self-hatred: it implies that’s it’s okay for them to judge people and for people to judge them, which reinforces their tendency to judge themselves.