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Â 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'Â 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)Â 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.