ETYMOLOGY
from blunder, taken in sense of ‘blunderer’ + -kin
EXAMPLE
“…I vtterly despaire of them, or not so much despaire of them, as count them a paire of poore ideots, being not only but also two brothers, two blockheads, two blunderkins, hauing their braines stuft with nought but balder-dash, but that they are the verie botts & the glanders to the gentle Readers…”
From: Haue with you to Saffron-Walden; or, Gabriell Harueys Hunt is Vp
By Thomas Nashe, 1596
Lexicophilia 