1 Kings 22:28 - But Micaiah replied, “If you return safely, it will mean that the Lord has not spoken through me!” Then he added to those standing around, “Everyone mark my words!”
Matthew 26:29 - "....Mark my words."
I guess the saying came from a time when people literally "marked" (recorded) words onto a tablet, as as a form of official documentation. This was done so that people could witness to a certain event or fulfilment of a prophecy or command. In today's language we could use it as a form of challenge, saying to people things like: "If you don't do this, then you will end up in trouble. Just mark my words!"
Marking words, could also be done in a helpful way, as we pass on important advice to loved ones or to those in our communities. If you wanted to pass on some very important words to a friend or loved one, so that they would remember them (or mark them), what would you say?
Living in Grace