An Etymological Dictionary of the German Language/Annotated/Schnapphahn
Schnapphahn, masculine, ‘highwayman,’ from the equivalent late Middle High German snaphan; yet it seems that the word signified originally a sort of musket, although this meaning is first recorded at the end of the 17th century, and hence is later than ‘mounted highwayman,’ which occurs even in the 15th century; the signification ‘musket’ was afterwards transferred to the man armed with such a weapon. Compare Dutch snaphaan, ‘gun, musket, bandit.’