This was an old proverb. Abel, like Teman, and some other places, was once famous for the wisdom of its inhabitants 1 Kings 4:30-31. The wise woman was herself a remnant of this traditional wisdom.
Related Commentaries of 2 Samuel 20:18
2 Samuel 20:18
18 Then she spake, saying, They were wont to speak in old time, saying, They shall surely ask counsel at Abel: and so they ended the matter.