"Catch flat-footed" is an idiomatic expression used to describe a situation where someone is caught off guard or surprised. There are several synonyms that can be used to convey a similar meaning, including "catch unawares," "catch off guard," "take by surprise," "startle," and "astonish." Each of these phrases implies that someone was not expecting a situation or outcome and is suddenly taken aback. Other related phrases include "caught in the act," which refers to being caught doing something undesirable or illicit, and "blindsided," which suggests being hit unexpectedly by a sudden turn of events. Overall, there are many ways to express the concept of being caught flat-footed, depending on the context and degree of surprise involved.