When you say that something knocked your socks off, what you mean is that you found it to be really exciting or good.”
“In other words, it pleased you very much. You were extremely pleased or happy with it. You were just blown away by the experience.
“You should see Narender’s new car. It’ll knock your socks off.”
“The new restaurant is a very small place. The decor isn’t great, but the food will simply knock your socks off.”
“You must try this new brand of ice cream. It’ll knock your socks off.”
“It’s also possible to say ‘blow the socks off someone’. When the little girl began to sing, she blew the socks off everyone.”
Source: “The Hindu” – Know Your English Column – March 04, 2008