There are certain things that are useless in life. A screen door on a submarine, a picture window in tank, an honesty pledge by a congressman - and here we learn that a proverb in the mouth of a fool is another.
A Proverb in the mouth of a fool is a complete waste of time and energy. A fool wants nothing to do with wisdom - and this word for fool indicates that he is immovably opposed to God and the morals that come from knowing God. The fool doesn't care about the future. He only cares about his own immediate desires. He thinks that God's Word and ways are stupid and a burden to live by - and rejects them for whatever his lusts are telling him to do.
The example here to compare how useless it is to have a proverb in the mouth of a fool is that of how the legs of the lame are useless to them. They cannot walk and cannot move. Therefore their legs are useless to them. They just hang there and have no purpose any longer. This is a tragedy - but then again so is the proverb in the mouth of the fool. He may say one thing - but his lifestyle militates against it - and he desires something altogether different.
When you hear a proverb being spoken by a fool - you can listen to the wisdom of the Proverb - just don't expect that wisdom to be carried out by the fool who speaks it.