In Matthew 12:34-36 Jesus is speaking and he reminds us that the words which come out of our mouth reveal what's truly in our heart.
"You brood of vipers, how can you who are evil say anything good? For out of the overflow of the heart the mouth speaks. The good man brings good things out of the good stored up in him, and the evil man brings evil things out of the evil stored up in him. But I tell you that men will have to give account on the day of judgment for every careless word they have spoken. For by our words you will be acquitted, and by your words you will be condemned."
So as you and I read this we need to search our hearts and ask ourselves, "Just what kind of words come out of my mouth?" "What kind of words do I think about saying but don't?" Those secret words we think in our head also reveal our heart. All those words spoken and unspoken reveal our true heart.
Just being careful not to speak those words in our heart isn't the way to clean our heart up. We need to ask the Holy Spirit which indwells us to fill us with new attitudes and motives. Then the true source of the words meaning our heart will be cleansed.
"Search me, O God, and know my heart; test me and know my anxious thoughts, See if there is any offensive way in me, and lead me in the way everlasting."