According to mythology, at one point Nuada was the king of the Tuatha De Danaan. However, during the First Battle of Magh Tuiredh (which was fought by the Tuatha De and their rivals, the Fir Bholg), Nuada's arm - or hand - was severed from his body. This unfortunate wound immediately disqualified the god from the rank of king, and so he was forced to abdicate (there were obviously very strict requirements that leaders be free of physical blemish). It was the clever god Dian Cecht who in part solved this problem, by making a silver arm to replace the one that Nuada had lost. And it was at this time that the former king received the title "Nuada Argetlamh", or "Nuada of the Silver Hand".
Although Nuada eventually regained his kingship (in large part because of his new metal arm or hand), he first had do battle with the unpopular king who had taken his place, Bres. This conflict - between Nuada and Bres - led to the Second Battle of Magh Tuiredh. In the course of the preparations for war, Nuada made the acquaintance of Lugh, and decided to relinquish his position so that Lugh could become leader of the Tuatha De Danaan.
Nuada was known as Nudd of the Silver Hand in Welsh mythology.
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Dictionary of Celtic Mythology
This book is a great source for information about Celtic mythology! Described as both a "who's who" and a "what's what", this reference book features entries on the important gods, heroes, and other characters from Celtic myth and legend. And there is a good entry on the god Nuada.
If you want more recommended resources for information about Celtic myths, visit the books section - it lists books about mythology, art, literature, and more.