It means to surprise or to shock. As a passive form (to be shocked or surprised): --- "My neighbors were thrown for a loop when they found twenty dogs in their apartment." -> "My neighbors were surprised when..." "Many people were thrown for a loop when the famous rock band cancelled their concerts." -> "Many people were surprised when..." "I was thrown for a loop when she told me that she hated math - she was always so good at it." -> "I was surprised when ..." "When I heard the news, I was thrown for a loop." -> "When I heard the news, I was surprised." As a transitive verb (to shock or surprise someone) - --- "He quit his job today. He really threw me for a loop." -> "He quit his job today. He really surprised/shocked me." "I threw my mom for a loop when I told her I didn't want to go to college." -> "I surprised my mom when ..." "They had thrown us for a loop by telling us that our bank account did not exist." -> "They had surprised us by ..."