It’s simply merry Christmas but in a funny type way or if you are excited and just want to follow the trend merry chrimuhh as shown in a more funny way of the regular saying “Merry Christmas”.
I don't buy the schmegegge about Morty sleeping with Moira.
His version of the story was pure schmegegge.
The whole schmegegge was made up to get Liz a little bit of attention.