Knock your socks off
The "socks" began as "spots." Most word experts agree that this phrase, meaning "to greatly impress," has its roots in sharpshooting. In the 19th century, American marksmen would shoot the "spots" (clubs, diamonds, etc.) off a playing card nailed to a tree. "Spots" became "socks" when language users forgot the origin of the expression.
Centuries ago, you'd simply say you were "whelmed," if you were overcome by something. "Whelm" has its roots in Middle English sea parlance. It meant to capsize or turn a vessel upside down. Before long, it grew to mean that the whelmed vessel was covered with water. "Over" was added to make the situation seem more dire, perhaps.
These terms for first and second teams owe their origin to the chief sport of the Middle Ages: archery. An archer's success lay in his use of a good bowstring. His favorite was his first string; his backup was his second. A man who carried "two strings to his bow" was well prepared.