Final answer:
The word "backbone" means the main or central supporting element. In the sentence, it refers to cotton being the main driving force behind the Southern economy.
Step-by-step explanation:
The author of the sentence uses the word "backbone" to mean the main or central supporting element. In this context, it refers to cotton being the main driving force behind the Southern economy.
Cotton production was the primary economic activity, and the profits generated from it were substantial.
Slavery provided the free labor necessary for cotton production, making it the backbone of the Southern economy.