The phrase you’re referencing comes from the Bible, specifically from the book of Matthew 24:42 (King James Version). The full verse reads:
“Watch therefore: for ye know not what hour your Lord doth come.”
This passage is a part of Jesus’ discourse about the end times, where He emphasizes the importance of being vigilant and prepared, as the exact time of His return is unknown. It’s a reminder to live with awareness and readiness for spiritual matters.