To let your hair down is to relax completely and stop behaving formally. Let your hair down for once.
The picture is literal, from an age when women pinned their hair up in public. To let your hair down at a work party is a decision several people regret every December.
1phraselet your hair downTo relax completely and stop behaving formally.
“He never once let your hair down in front of the staff.”
let your hair down is a phrase.
Practised in Idioms