From the Latin arcanum, meaning "secret", an Arcanist is someone who has access to, or professes to have access to, secret knowledge.
Historically, "Arcanist" was the term applied to persons in 18th century Europe who claimed to know the secret to making porcelain. For centuries the secret to making porcelain was unknown to Europeans, and the only way to get this hard, durable, beautiful ceramicware was to import it, at great expense, from China. This meant that anyone who could unlock the secret could stand to make a large fortune.
After several false starts, the true secret of porcelain was finally unlocked by German alchemists Ehrenfried Walther von Tschirnhaus and Johann Friedrich Böttger in 1708, and a full scale state-run porcelain factory was established at Meissen in 1710. The process for making porcelain, however, remained a closely guarded state secret in Germany and later in France for over a century.
Some workers at the Meissen factory however, managed to escape with knowledge of the process, which they then began surreptitiously marketing across Europe. These were the Arcanists. But these possessors of real knowledge were far outnumbered by the "Pseudoarcanists" who claimed to have knowledge of the secret, and would be happy to sell you "porcelain" which turned out to be a brittle forgery.
On Everything2, "Arcanist" is a quasi-secret level title for Level -1. Achieving Level -1 apparently requires achieving negative one million XP, which used to be theoretically possible using punch thyself (although it would have required a ridiculous amount of time and tenacity). So far the only user to have ever successfully become an Arcanist on E2 is thefez.