keep one's guard up

keep one's guard up (third-person singular simple present keeps one's guard up, present participle keeping one's guard up, simple past and past participle kept one's guard up)

1. To be careful and alert to the possibility of danger.

   Synonyms: have one's guard up, be on one's guard

   Antonyms: lower one's guard, let one's guard down

• “keep one's guard up”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.