Possum

google

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early 17th century: shortening of opossum.

wiktionary

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Derived from opossum by apheresis.

etymonline

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possum (n.)

American marsupial mammal, 1610s, shortened form of opossum (q.v.). It is nocturnal, omnivorous, and when caught or threatened with danger feigns death; hence the phrase play possum "feign death when threatened," attested by 1822.