I agree that etymology can be useful and fun. On the other hand, it’s not always relevant. Just as with current English usage, writers and speakers generally use words with little or no consciousness of the root meanings of the words. Of course, entailments can drive or frame meaning, conscious or not. Literate Chinese may tend to be a bit more conscious of etymology, however, because of the graphic nature of the written language. In Chinese and Japanese, there are many folk etymologies of words that are in fact patently false with regard to the actual evolution of the characters in question, but which have developed independently over time as explanations for the composition of given characters simply because they help people to easily remember the written character. Chinese also like to play word games that are sort of like graphic riddles, where you’re supposed to guess a character based upon a fanciful description of a little scenario involving the graphic elements.
As for the character “song,” it is composed of one element that means “long hair,” (which is in turn composed of discreet elements meaning “long” and “hair”). Combined, this “long hair” graphic element appears in various modern characters for the hair on one’s head, or facial hair, etc. I’ve seen one etymological explanation for a character based on that element, the character “xu,” which can mean either “beard,” or “should, must.” The explanation for the “should/must” meaning is kind of funny. Supposedly, in traditional Chinese society, a man didn’t grow a beard until he was married and in a position of authority. That is, until he had the authority to tell others what they should or must do. Is this accurate? Who knows?
In any case, “song” contains the element depicting “long hair,” plus a phonetic element that indicates its pronunciation, which here is a graph for a pine tree, pronounced “song.” I don’t know for a fact, but I think the “loosen” meaning of song comes from the image of long hair that hangs freely, and is not bound up. Prior to the Qing (Manchu) dynasty, Chinese men typically had long hair, but they usually wore their hair in a topknot, especially in formal circumstances. So, when they took their hair out of the topknot, it was “loosened.” There could be an attendant entailment here for another modern meaning of song, which means “to take it easy,” or “catch a break.” One might imagine that in less formal situations, when hanging out, strumming on a lute and drinking wine, the topknot was optional. So at times like these, you could “loosen up naturally,” and be at ease. Again, I don’t know if this is accurate. Who knows?
The “pine tree” element that gives our “song” character its pronunciation has entailments that, to me, could have some bearing on the idiomatic taiji meaning. A pine tree has an upright bearing. Its trunk is straight. Its branches don’t droop; they are buoyant and springy. (The pine, as an evergreen, also has a metaphorical entailment of longevity in Chinese culture.) Does this entailment have any validity in the taiji context? Who know? By the way, the simplified version of the “loosen” song character that has been in use on the mainland since the ‘50s no longer has the “long hair” element present in the traditional character. Instead, the character used for the meaning “loose, loosen, relax” is the same one used for “pine tree.” People who are familiar with both traditional and simplified characters (and most American students of Chinese have to learn both) tend to be biased in favor of traditional characters, because the simplified forms have a lot of the etymological detail stripped out of them. Also, the simplification scheme dictated that many characters do double duty, as in the case of this “song” character. Sometimes it works well, other times, not so well. One has to wonder what the rationale is for the pine tree character being used for “loosen, relax,” etc. Is a pine tree relaxed? You would have to ask the tree.
Was there a particular classic text you had in mind? From what I can recall, only one of the five “core” taiji classics uses the word “song.” That is the Mental Elucidation of the Thirteen Postures, where it occurs a couple of times as follows:
“The energy (jin) seems loosened (song) yet not loosened; about to expand, but not yet expanding.”
“The abdomen is loosened (song) so that the qi gathers into the bones.”
Most interesting in light of the discussion we’ve been having, I think, is the first example. This idea of “seems loosened but not loosened” (si song, fei song) may capture the real feeling of what we’re talking about. In taiji movement there is a loosening of the joints and tissues—an absence of tension or rigidity, but at the same time, one’s posture is upright, and one’s limbs are extended. It seems loose, but is not loose. It’s not something that can be expressed in a cut and dried manner.