Francophonie· easy
avoir une peur bleue
“to have a blue fear”
Meaning
To be absolutely terrified of something
Explanation
The stronger cousin of just being scared, this is proper fright.
Example
Il a eu une peur bleue quand il a vu l’ombre bouger dans le couloir.
He was scared stiff when he saw the shadow move in the hallway.
Origin
First shows up in the French press in December 1846. One theory ties the blue to the 1832 cholera outbreak in Paris, when victims' skin turned a telltale bluish colour.
Source