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.

The origin of this phrase goes back to mythology and perhaps the Christian cross. It has an interesting history, read about it here . We use the phrase today to indicate we are 'warding off' evil spirits or negativity, to bring about good luck in a sense,