"I wanted to find out what “rest” meant in the Old English language. It did not mean 'sleep.' It meant 'make' or 'keep'—'God Make You Merry, Gentlemen.' But there was something else I wanted to find out. I figure, 'If the word ‘rest’ meant something different, maybe, ‘merry’ meant something different. Maybe, it wasn’t Robin Hood and his ‘happy’ guys out in the forest. Sure enough, it wasn’t. 'Merry'—in the 1500’s and 1600’s, when this song was written—meant 'mighty'—'mighty' or 'great.' Think of the song this way: 'God make you mighty, gentlemen. Let nothing you dismay. Remember Christ your Savior was born on Christmas Day.' So, rather than saying: 'Happy Christmas,' or, 'Merry Christmas,' to each other this year, we need to be saying: 'Mighty Christmas,' 'Have a Mighty Christmas,'—'Have a powerful Christmas,'—'Have a great Christmas!' That’s what that song means. That song, which we kind of almost discredit in modern-day usage, has a very, very powerful meaning that we’ve lost."
You can read more about Ace Collins here.