catch sb off guard

catch sb off (their) guard (surprise someone) — застать врасплох

be caught off guard (be surprised, unprepared, not expecting) — оказаться не подготовленным к неожиданности; быть застигнутым врасплох

Example: I was really caught off guard by your visit. I didn't even have anything to eat or drink. — Я к вашему приходу совершенно не готова была. У меня даже ни поесть, ни попить ничего не было.

Example: "Why on earth did you tell him that?" " He caught me off guard!"

see also
[take sb aback]
[catch sb unawares]
[застать врасплох]
[с поличным ]
[catch sb in the act]
[catch sb with one's pants down]