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from pro- + nephew, after French pronepveu (1486 in Godef.), Latin pronepōtem pronepot
From: De Verborum Significatione:
The Exposition of the Termes and Difficill Wordes
Conteined in the Foure Buikes of Regiam Majestatem, And Uthers.
Collected and Exponed Be M. John Skene, 1681
Note: 'pronepuoy' is an old form of 'pronephew'