"Catch
someone off guard" is an idiom that means to surprise or shock someone by
doing something unexpected or by catching them unprepared.
The
phrase is often used in informal contexts.
Here
are some short interesting sentences using this idiom:
1.
Sandra’s
sudden question in class caught students off guard and they didn't know how to
respond.
2.
The
unexpected rain shower caught us off guard, and we got completely drenched.
3.
The
sudden loud noise caught me off guard and made me irritated.