|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)