It's the usage that originated with Lewis H. Morgan. Engels used it in "The Origin of the Family, Private Property and the State", which Wikipedia has a good summary of:
> Barbarism – the period during which man learns to breed domestic animals and to practice agriculture, and acquires methods of increasing the supply of natural products by human activity.
> Barbarism – the period during which man learns to breed domestic animals and to practice agriculture, and acquires methods of increasing the supply of natural products by human activity.
https://en.m.wikipedia.org/wiki/The_Origin_of_the_Family