special boast
The 'special boast' is the Dutch proverb that the older a Dutchman is, the stupider he is,
Hoe ouder, hoe botter Hollander.
By claiming a special relationship with the Dutch, Folly is of course being ironic at Erasmus's expense.
Note to My Achievements And Attributes which is Part 2 of Folly Speaks from "The Praise Of Folly"