21 “Also take no heed unto all words that are spoken; lest thou hear thy servant curse thee:”
Be careful about being over affected by what people say about you; people bless and curse indiscriminately. If you hear, by accident, something someone said about you then let it roll off you. Bear in mind you probably said (or thought) at one time or another troubling things about others (See next verse as well Ecclesiastes 7:22). Whilst the scriptures do talk quite a bit about a good name and even of being well spoken of by others the truth of the matter is that some will have very negative feelings about you (Cain hated Abel because Abel’s works were righteous and Cain’s were evil, this is the natural animosity that exists between the righteous and the wicked). You must not let these opinions and words spoken against you have to much affect on you.
22 “For oftentimes also thine own heart knoweth that thou thyself likewise hast cursed others.”
Building upon the last verse (V:21) Solomon is saying, "Be realistic, have you yourself only spoken positive things about others?” If you have said things without much forethought then is it not reasonable to expect others to do the same. And so do not take their words to heart. That being said we are told in the scriptures to let our conversation be chaste; let it be pure and without sin and hypocrisy. The words of our mouth are so important with it we praise and bless the Godhead let us not use it to tear down His creation.