We use words to communicate observations and ideas. But it is valuable to remember that words are imprecise. The same words don't always convey the same meaning to everyone.
The author was reminded of that limitation when pondering the phrase letting down ones hair. Presumably the expression was coined during an age when people fastened their hair up on their heads into formal, static hair arrangements. After formalities were unneeded, a person could relax by
letting down their hair. This phrase also came to refer to whenever a person shed formal pretensions and allowed their true nature to be revealed. For many people, this may encompass their intended meaning when using this phrase.
But recently the author realized there may be additional implications in the phrase. The author's hairline has receded and top hair thinned as the author aged. As efforts to hide such aging became more vain, the author abandoned those efforts, deciding to just shave his hair at about a centimeter length. But the short hair provided another awareness previously missed. The short hair, which rarely moves, is now very sensitive. Each hair is associated with a nerve capable of detecting motion of the hair. That sensation is of course missing if hair is shaved clean.
But what was previously unrealized, long hair which constantly moved apparently tired the nerves, resulting in reduced sensation. Presumably long hair fastened into a formal, static arrangement rarely excites the nerves. Thus letting down one's long hair to freely move with rested nerves results in increased sensation. To someone familiar with this, the expression
let down ones hair could also imply increasing ones sensitivity.
Of course this is but a single example of how words or phrases have different subtle meanings for people with different past experiences. As a result, words spoken or written with one intended meaning often are understood a bit differently by a reader or listener. While the difference is often trivial, is can occasionally cause profound misunderstandings. Because verbal language does not have the same precise meaning for all is one of the reasons why scientists prefer to use mathematical equations when trying to convey precise meaning.