Knock someone’s socks off

In the mid-19th century, this expression was originally connected with brawling, when fist fights could be so fierce that some protagonists would literally lose their shoes and socks in a fight. Since then, from the late 19th/early 20th century, it has migrated to a softer, more genteel meaning, to surprise, amaze or delight someone. See also work one’s socks off.