When I was looking at the meanings of pin 牝, the Daodejing came to mind. Pin 牝 occurs in The Judgment of hexagram 2 (and in the Judgment of hexagram 30, but that does not concern us here):
利牝馬之貞.
“Favourable in a consultation for a mare”. If the oracle is consulted to know whether a mare (pinma 牝馬 = ‘female horse’) is carrying a young, then this is a favourable answer: the mare is carrying. In metaphorical sense this means that this hexagram is favourable for all female matters.
Pin 牝 generally means ‘female, feminine’ (雌性的獸類，引申為雌性的), and this is how it appears in the oracle bone drawing of this character: an animal combined with the sign for ‘woman’, in other words a female animal. But pin also has another meaning, namely ‘gully’ or ‘valley’ (溪谷). Already in ancient times is hexagram 2 linked with the feminine, and it is interesting to see that the meaning ‘valley’ makes a connection with other texts like the Daodejing 道德經, where we also find ‘valley’ combined with the feminine. In chapter 6 we read:
谷神不死, 是谓玄牝.
“The valley spirit never dies, it is called ‘the mysterious feminine’ “.
In old books like the Da Dai Liji 大戴禮記, ‘Notes regarding rituals, from Dai Senior’, it is said: 丘陵為牡，谿谷為牝 – “hills are mu 牡 masculine, valleys are pin 牝 feminine”. A link with the genitals is obvious. Earlier another meaning of pin 牝 was ‘the part of a lock that receives the key’ (古代鎖具容受鍵的部分). In the Liji 禮記, the Book of Rituals, we therefore read “the key is masculine, the lock is feminine” (鍵，牡; 閉，牝也).
Although the name of hexagram 2, kun 坤, appears for the first time in the Yi en therefore we cannot look at other literature that can give a hint about the meaning of this character, we could speculate that it’s original meaning is ‘valley’. The character consists of two parts, 土 for ‘earth’ and 申 for ‘streched’ (according to Karlgren, GSR 421a). This could refer to a valley, and we can add that valleys are often fertile (= feminine) because of the rain water that flows to it. I am not very fond of speculating so I’d better stop here, nevertheless I found it worth to mention it in my diary.