Plural of derby: either a type of stiff felt hat with a rounded crown, or a racing competition (especially horse racing), or a sports match between rival local teams.
Named after the Earl of Derby, who in 1780 established a famous horse race in Derby, England. The hat was named after this race because attendees wore them. The meaning expanded to any local sports competition.
The derby hat became so iconic with the sport that when people see that distinctive round dome shape, they instantly think 'fancy race day'—that's how successful branding works, and this one lasted over 200 years.
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