From Wiktionary, the free dictionary
Inherited from Proto-West Germanic *murgiþu (“briefness, brevity”); equivalent to myriġe (“merry”) + -þ (“-th, -ness”)
myrgþ f
- mirth
Declension of myrgþ (strong ō-stem)
- Middle English: myrthe, merth, merthe, merþe, mirthe, mirth, murhthe, murþe, myrth, myrþe, merhð, mirhð, mureȝþ, murehþ, murcð, murȝþ, murhd, murhð, muruhð (Early Middle English)