Definition of the word defile means to make something or someone unclean or impure or to corrupt the purity or perfection of something or someone. It could also mean to be debased or to make something or someone filthy or dirty. If you polluted a river then you'd defile a river, like with sewage or something similar. This is not that much different from the biblical definition of defile
Yahushua Jesus was rightly concerned with the heart saying, "it is not what goes into the mouth that defiles a person, but what comes out of the mouth; this defiles a person" (Matt 15:11). He said this because it is "what comes out of the mouth proceeds from the heart, and this defiles a person" (Matt 15:18) and not what goes into the mouth. What is it that Jesus was referring when He said "but what comes out of the mouth [that] defiles a person?" It was the words we use because our words reveal what is in our heart and if we are defiled before God it is because our words betray us for out of the mouth speaks what is hidden in the heart.