We found 2 dictionaries that define the word have one's guard up:

General (2 matching dictionaries)

1. have one's guard up: Merriam-Webster
2. have one's guard up: Wiktionary

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