Green’s Dictionary of Slang

buttermouth n.

[nationalist stereotyping]

a Dutchman; also attrib.

[UK]A. Boorde Introduction of Knowledge (1870) 147: I Am a Flemyng, what for all that, [...] ‘Buttermouth Flemyng,’ men doth call me .
[US]Maledicta VII 24: Buttermouth and butterbox are old slurs on the Dutch.