Apparently, the phrase first appeared in the mid-19th century meaning 'to beat or vanquish someone thoroughly'. At first it was used, to mean to win in a knock-down fist-fight so savage, that the loser might expect not only to lose his shoes in the fight but also his socks as well!

The number of brawlers who actually lost their socks was probably pretty small but a threat 'to knock your socks off' was one of a number of such phrases popular at the time including, 'knock your lights out' and 'knock you into next week'.

Among folks who were not inclined to physical combat, to 'knock someone's socks off' was soon adopted in a more general sense of 'to win decisively', and one might 'knock the socks off' one's opponents as well in a game of cards or in the boxing ring. From there the phrase mutated a bit more and 'to have one's socks knocked off' came to mean 'to be amazed, delighted or very impressed.'

For the service and products I provide, when it comes to managing your data I'll Knock Your Socks Off (.com) too!