yammer v.

1821, in a German context, "a hangover," American English colloquial, from German Katzenjammer "hangover" (18c.), also figuratively, in colloquial use, "remorse of conscience, vow to mend one's ways," literally "wailing of cats, misery of cats," from katzen, combining form of katze "cat" (see cat (n.)) + jammer "distress, wailing" (see yammer (v.)).

Pleasure can intoxicate, passion can inebriate, success can make you quite as drunk as champagne. The waking from these several stages of delights will bring the same result--Katzenjammer. In English you would call it reaction; but whole pages of English cannot express the sick, empty, weary, vacant feeling which is so concisely contained within these four German syllables. ["Blackwood's Edinburgh Magazine," August 1884]

Katzenjammer Kids "spectacularly naughty children" is from the title of the popular newspaper comic strip (based on the German Max und Moritz stories) first drawn by German-born U.S. comic strip artist Rudolph Dirks (1877-1968) in 1897 for the "New York Journal." Hence, katzenjammer in the sense "any unpleasant reaction" (1897). The strip was temporarily de-Germanized during World War I:

"THE SHENANIGAN KIDS" is the new American name for the original "Katzenjammer Kids." Although the original name and idea were pure Holland Dutch, some people may have had the mistaken impression that they were of Germanic origin, and hence the change. It is the same splendid comic as in the past. [International Feature Service advertisement in "Editor & Publisher," July 6, 1918]