means lively, cheerful fun and laughter, usually shared among people in high spirits

from From 'merry,' which traces back to Old English 'myrige,' meaning pleasant or agreeable. The '-ment' suffix arrived later, borrowed from French and Latin, as a tidy way to turn a feeling into a thing. So 'merriment' is essentially 'the state of being merry' — though that older 'myrige' carried a softer sense of agreeableness, possibly related to a Germanic root meaning 'short' (as in time made to feel brief by pleasure). The boisterous laughter we now hear in the word grew louder over the centuries.