What does "off guard" mean?

Also, off one's guard. Not watchful, easily surprised. It is often put as catch (or be caught) off guard, meaning “take (or be taken) by surprise.” For example, The securities analyst was caught off guard by that financial report, or With any luck the boss will be off guard when I come in late. [Late 1600s] The antonym, on guard or on one's guard, meaning “watchful or prepared, especially to defend oneself,” was first recorded in 1577. For example, In this crowd we must be on guard against pickpockets, or I'm always on my guard when I'm asked how I voted.