We found 2 dictionaries that define the word let one's guard down:

General (2 matching dictionaries)

1. let one's guard down: Merriam-Webster
2. let one's guard down: Wiktionary

Definitions from Wiktionary (let one's guard down) ▸ verb:  Synonym of lower one's guard ▸ Words similar to let one's guard down ▸ Usage examples for let one's guard down ▸ Idioms related to let one's guard down ▸ Wikipedia articles (New!) ▸ Words that often appear near let one's guard down ▸ Rhymes of let one's guard down ▸ Invented words related to let one's guard down