hair down?

[okok]

In a text about a less rigorous book by some well-known thinker the author says that this text "lets us see him with his hair down". Is this a known idiom? Does it simply mean what it seems to mean or is there something special to be understood here?

[twostep]

In a text about a less rigorous book by some well-known thinker the author says that this text "lets us see him with his hair down". Is this a known idiom? Does it simply mean what it seems to mean or is there something special to be understood here?

to let your hair down, relax, be at ease

[okok]

Thank you