Matthew 12:34 (Berean Study Bible)
For out of the overflow of the heart, the mouth speaks.
There are things that we utter in the heat of moment and may not really mean. But, there are strong words, intents, desires and feelings that we try to carefully bury deep in our hearts that will eventually come out of our mouths. Even the most civil and diplomatic tongue will find it difficult to restrain what's truly in the heart from coming out. The only true safeguard is not taming the tongue but allowing the Holy Spirit to cleanse our hearts.