Zoo

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mid 19th century: abbreviation of zoological garden, originally applied specifically to that of Regent's Park, London.

wiktionary

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Clipping of  zoological garden, now the usual form. See  zoology. 

etymonline

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zoo (n.)

c. 1847, short for Zoological Gardens of the London Zoological Society, established 1828 in Regent's Park to house the society's collection of wild animals. The first three letters taken as one syllable. "From a mere vulgarism, this corruption has passed into wide colloquial use" [Century Dictionary]. Slang meaning "crowded and chaotic place" first recorded 1935.