Etymology
Late 17th century, of unknown origin;
* Possibly related to Middle English bemothered (“confused, perplexed”), itself probably related to Old English mēþe (“exhausting, troublesome”), Old Norse móðr (“angry, frustrated”), Middle Dutch moeden (“to tire, exhaust, disturb, trouble”).
* Possibly allied to Welsh moedro (“to worry or bother”), though according to the GPC, the latter is borrowed from English.
* Alternatively, possibly tied to Welsh meidda (“to beg for whey”), from maidd (“whey”).
* Perhaps from Welsh meiddio (“to dare or venture”), a variant of beiddio (“to dare”).
The "dd" in Welsh corresponds in sound to the "th" in mither, and English also has moider and moither.