Plural of kook; strange or eccentric people, or people with crazy ideas.
American slang from the early 20th century, possibly from 'cuckoo' (the bird associated with madness) or from Dutch 'kook' (cook). The exact origin is debated, but the connection to oddity is clear. Became popular in 1950s-60s surfing culture.
In the 1950s-60s, 'kooks' specifically meant inexperienced or reckless surfers, and surfers still use it that way today—it's slang that actually had a specific subculture origin and then spread into general speech!
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