Today's idiom was uttered during fights, I'm thinking when an especially powerful hit provided the needed knockout in a fight. But it became a popular phrase in the mid-1960s when Pepsi-Cola wanted to increase the popularity of a southern drink called Mountain Dew. It was a regional drink that Pepsi thought had potential to become popular nationwide. The first ad campaign promised to "knock the socks off" anyone who drank it. ( source ) The phrase today is a positive one meaning that someone who has their socks knocked off has been surprised or astonished.
"We realized that the important thing was not the film itself but that which the film provoked." Fernando Solanas