What's the latest English word you learned thanks to WK

[Saida]

I learned ‘tricorne’ just now.

ハリー・ポッターと賢者の石 - chapter 5

I’m wondering if I’m missing something cultural here. In the letter from Hogwarts, Harry is asked to bring a 普段着の三角帽, but no matter how I read it - that’s a . I was so unsure about it that translation I had to check in my English copy of the book, which calls it , which at best has only 一角…