The expression “to get one’s Dutch up” means: To get one’s dander up; to arouse one’s temper.
The reference is, of course, to the Pennsylvania Dutch, to the people of Germanic origin who, in the early seventeenth century, fled from continued religious persecutions in the Palatinate, chiefly, and brought their brands of Protestant faith into the sanctuary provided by William Penn in eastern Pennsylvania.
They were a peaceful people, these paternal ancestors of mine. And they practiced their religion.
Accordingly, they were slow to anger, keeping their tempers under subjugation.
Nevertheless, so the historian Bancroft tells us, although representing only one-twelfth of the population at the time, the Pennsylvania Dutch composed one-eighth of the army in the Revolutionary War.
And, from personal experience, we can vouch for the fact that some of their descendants, at least, had flaring tempers, although they might not let “the sun descend upon their wrath.”