Naba' (H5042) describes the outpouring or abundant flowing of something β water from a spring, speech from the heart, or wisdom from the mouth of the righteous. It appears in Proverbs to describe how the righteous pour forth wisdom while the wicked utter crooked things. The image is of something so full it naturally overflows.
The verb is used metaphorically in Psalm 78:2 and Proverbs 18:4. The heart that is full of wisdom cannot contain it β it bubbles up. This image underlies the NT concept of the Holy Spirit as living water springing up (John 4:14). What fills the heart determines what naba' β overflows β from the mouth.