The word 'merry' means cheerful, lively or high spirited. The word 'Christmas' means the mass or worship service of Christ. So to wish someone a Merry Christmas basically means that wish that someone would have a lively, high spirited worship experience of Christ.
The next time you wish someone a Merry Christmas, decide that you too will have a lively, high spirited worship of our Lord and Savior Jesus Christ. Live out Psalm 100 by, 'Making a joyful noise unto the Lord, serving the Lord with gladness, and coming before his presence with singing.' When we joyfully celebrate in worship the birth of Jesus Christ we are truly having a Merry Christmas.
Take the time to celebrate this Christmas season with the ones you love. Celebrate with gladness that Christ came into this world to be our salvation. Share the reason for the season with someone who doesn't know what Merry Christmas really means. And be sure to have a Merry Christmas.