The etymology of the word merry underwent a myriad of changes in Middle and Old English, undergoing alterations such as merrie, mery, merie, mirie, myrie, murie, merige, myrige, mirige, myrege, and... you get the idea. Clearly the word was used a lot back then; indeed, N-grams show it being most commonly used in the 1600s. Anyway, through all this time, it still meant "jolly" or "pleasant", but as we move further back to the reconstructed Proto-Germanic root murguz, it meant something more like "brief" or "short". This transition highlights the quality of happy moments to be fleeting, or of merry moments to be short. The final root to be reconstructed is that of the Proto-Indo-European mreg, meaning "short", the root of words from brevity to mirth. Going further back than that is impossible, but on a more recent and unsurprising note, Google search interest for the word "merry" skyrockets every December.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd