From Old English (before 12th century), from Proto-Indo-European '*mreǵhu-' ("short, brief"), from PIE *mreǵhu- ("short, brief").
Amusement, especially as expressed in laughter; gladness and gaiety accompanied by laughter.
From Old English 'myrgþ' (mirth, joy, pleasure), from 'myrge' or 'merge' (pleasant, agreeable, delightful), from Proto-Germanic '*murguz' (short, brief), from PIE root *mreǵhu- (short, brief). The connection between 'short' and 'pleasant' is puzzling until you consider that the Germanic peoples may have associated brevity with lightness and pleasantness — something brief is light, something light is merry. The same root gives us 'merry,' making 'mirth' and 'merry' siblings. The word 'mirth' specifically
The etymological link between 'short' and 'merry' may explain the English expression 'short and sweet' — the association between brevity and pleasantness appears to be literally prehistoric, encoded in the Proto-Indo-European root that became both 'mirth' and the Greek word for 'short.'