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from boy (n.) + -kin
1547 - A dictionary in Englyshe and Welsh, William Salesbury - see below from E-NED
From: The First Part of the True & Honorable History, of the Life of Sir Iohn Old-castle,
the good Lord Cobham
...Formerly attributed to Shakespeare, who at first founded the character of Falstaff on Oldcastle. See bottom clipping for explanation of author and year.