The reason John Oliver is wearing this hat is that Donald Trump's name was Drumpf in Germany before his relatives moved over here. However, it might be important to note that most people from Europe (especially places like Germany east or even Switzerland to the east of there, often changed their names when they came here because of ethnic discrimination which was prevalent before about 1900 here in the U.S. The only thing you might be safe being up until then was English, and even then if you were seen taking a long time American's job you might be beat up or worse. So, name changes were quite normal up until at least 1920 or beyond, especially if you came through Ellis Island in New York on a ship to the U.S. for the first time to immigrate here.
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According to CNN Money, Oliver's “Donald Drumpf” segment from February (you can watch it here) has broken viewership records not just for Last Week Tonight but HBO in general with over 23 million YouTube views and over 62 million Facebook views, ...
‘Last Week Tonight’s “Donald Drumpf” Episode Has Broken HBO Viewing Records
Prior to the “Drumpf” segment, Oliver’s most-viewed video was his FIFA breakdown from 2014, which currently has over 12.5 million YouTube views. Last Week Tonight was pretty late with its Trump coverage compared to other late night shows — Oliver admitted to Stephen Colbert back in October that he “couldn’t give less of a shit” about Trump at the time: “It’s the 2016 election and it’s 2015 right now, so I don’t care until we’re in the same year as the thing that I’m supposed to care about.” Thankfully, the waiting paid off for Oliver and crew in a big way.