There appear to be a number of theories as to why the slang for the vagina (or perhaps more precisely, the vulva) is pussy. The most common derivation that I see in dictionaries and other resources is that it can be traced back to the Old Norse puss meaning "pouch" or "pocket". In that sense, it is not unlike the etymology of vagina which itself comes from the Latin vagina meaning "sheath" or "scabbard".
I've noticed a few people using the idiomatic phrase "prime of place" to denote the primacy of position in a group of things. For example, "My framed moustache-of-the-year certificate is given prime of place on the wall". While the idiom certainly appears to make sense, fits in quite well, and even sounds familiar, it is not really standard usage.
Somebody asked me earlier today why a penis is also called a cock. I assumed that it had something to do with the rooster (the male chicken) being called a cock. While this is true, there's a little bit more to tell. Here's a brief investigation into the matter.
According to Etymonline:
Slang sense of "penis" is attested since 1610s (but cf. pillicock "penis," from c.1300);
I was wondering earlier why the predominant color of Islam is green. This led me to wondering why it's also associated with emotions like envy and jealousy. Here are a few of the fruits of my dig deep down into the "emerald mine of knowledge" that is the Internet.
From Shakespeare's The Merchant of Venice: