In the 17th century a woman only let her hair down to bathe or brush, otherwise she had it pinned up. To have it down for any other reason the woman was considered uninhibited and/or wanton. ( source ) When we use it today its meaning is broaden to include either gender and its meant to act in a free/relaxed manner.

I looked in the mirror today at my hair free of product as the wind was picking it up, rather unattractively I must say, and saw threads of silver.  I know most people try to cover up their threads of silver but I am proud of them...and yes, grateful for them. Hey I've worked hard for these threads that are a symbol of life experience, parenting, marriage, relationships, settling into my own skin, etc. I'm a firm believer in what Proverbs says about these threads of silver . I'm grateful for all of the life that has brought me these threads of silver and I wear them proudly.