When it comes to expressing astonishment or surprise, the phrase "acc of f" might be recognizable as an abbreviation for "caught off guard". However, if you're seeking alternative synonyms for this phrase, there are several options available. Variations like "startled", "taken aback" or "stunned" can be used interchangeably to convey a sense of being unexpected or unprepared for a situation. Other synonyms include "bewildered", "flabbergasted", "shocked" or "amazed". Each of these phrases captures the essence of being caught off guard, providing a range of vocabulary choices to effectively communicate a similar feeling of astonishment or surprise.