What does goober mean in the Civil War?

Answer 1

In the southern states goober peas (peanuts) were commonly used as pig fodder. they mature under ground and pigs were simply turned out to root them up. As opposing armies passed through an area using a scorched earth policy in an effort to leave nothing useful to the enemy. Goober peas, being under ground, were commonly the only food left available in any sufficient amount to eat and doubtlessly saved many from starvation.

In southern slang calling someone a "goober" is the same as calling them a "peanut brain".