Proverbs 26:2
As a bird by wandering, as the swallow by flying, so a curse causeless shall not come. KJV
Although often negative, curses are sent for a reason. An example of causeless is found in Proverbs 23:29. In this verse the consequences are not without cause. A curse is the opposite of a blessing. It lowers a blessing to a lesser state.
In Judges 17 we read of a woman whose son stole eleven hundred pieces of silver from her and so she cursed the person who stole it. He heard her utter the curse and became afraid of the curse and returned the silver. Both of them believed in the power of the curse so she had a portion of the silver made into an idol and the son set up a shrine and made his son priest. They did all this because they believed this was the only way to nullify the curse. If we continue to read on into Judges 18 we will see how this curse eventually affected the whole tribe of Dan and, in the future, the ten tribes of Israel. Judges 18:30-31.
The reason they believed in the curse was because they did not believe in God. Judges 18:1 tells us why and this is what the curse ultimately revealed. Deuteronomy 11:26-28. Consider the verses in this study as homework. Be Blessed!