Some people are very reserved when they are in the company of others. When you tell a person to let his hair down, what you mean is that you want him to relax and start enjoying himself. You want him to speak his mind.
It was only after the completion of this last exam that Chetan let his hair down.
In the old days, women had to put their hair up in a hun during the daytime. The only time when they could literally let their hair down and themselves was when they were ready to go to be.
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