Is the word “chum” to mean friend a common word

WiseGeek.com says:

The term “the pot calling the kettle black” is usually used in the sense of accusing someone of hypocrisy. The origins of the phrase date back to at least the 1600s, when several writers published books or plays which included wordplays on this theme. Despite suggestions that the phrase is racist or nonsensical, the meaning is actually quite obvious when one considers the conditions of a medieval kitchen.

Typically, pots and kettles were made from heavy materials like cast iron to ensure that they would last and hold up to heat. Cast iron tends to turn black with use, as it collects oil, food residue, and smoke from the kitchen. Both pots and kettles would also have been heated over an open fire in a kitchen. As a result, they would have become streaked with black smoke despite the best cleaning efforts.

Since both are black, the pot calling the kettle black would clearly be an act of hypocrisy. The act could also be described by “it takes one to know one,” and it suggests a certain blindness to one's personal characteristics. There is another explanation for the term, involving the pot seeing its black reflection reflected in a polished copper kettle. In this sense, the pot does not realize that it is describing itself.

One of the earliest written instances of the phrase appears in Don Quixote, by Cervantes. The epic book was published in the early 1600s, and had a big influence on the English language. Numerous terms and idioms have their roots in Don Quixote, such as “quixotic” to describe an idealist. Shakespeare also played with the concept in one of his plays, as did many of his contemporaries. The phrase has been twisted and expanded over the centuries, appearing in forms like “pot, meet kettle.”

Some people believe that the phrase is racist, since it refers to the surface color of the objects involved. These individuals might want to keep in mind that in a modern kitchen, the idiom might be “the pot calling the kettle silver,” in a reference to the fact that many modern pots and kettles are often made from polished stainless steel. In this particular instance, skin color has nothing to do with the idiom, except in the sense that both of the objects involved are the same color.

Phrases.org.uk defines its meaning as:

The notion of a criticism a person is making of another could equally well apply to oneself

I expect all English speakers to understand sane and insane.

Although you might be able to find unsane in very large dictionaries such as the Oxford English Dictionary, I would not expect many English speakers to be familiar with it. I imagine most speakers would understand it as a novel combination of un- with sane (meaning "not sane"). I would also imagine that it might take a moment to understand--although I can imagine some circumstances where it would be readily understood, particularly when contrasted with sane and when the prefix un- is stressed:

Alice: So you're saying he's sane?
Bob: Well, he's not un-sane.

But I wouldn't expect many native speakers to invent this word. Prefixing un- to sane is probably blocked for most speakers by the existence of insane. I think that anyone who did (re-)invent this word would be suspected of wordplay.

Barrie England's answer gives the result counts for insane and unsane in The Corpus of Contemporary American English (COCA), which I'll repeat here:

   insane       3760
   unsane       2

I decided to check those two results. Neither uses unsane as discussed here; they're both references to the band Unsane. So it's really 3760 to 0, which is a pretty big ratio. Unsane is nonstandard and is essentially never used. Avoid, avoid, avoid.