Definition - The reconstructed ancestral language (i.e. the Ursprache) of the Indo-European family of languages. Purportedly, it was a living language that died out about 4,000 years ago.

Oxford English Dictionary -
The term's first citation (s.v. proto-) is from 1947:
"Linguists have reconstructed large parts of the vocabulary of *Proto-Indo-European."
(R. S. Wells in Word III. 15)

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