Clueful. Plugged-in. One of the cognoscenti. Also used as a general positive adjective. This term is not actually native hacker slang; it is used primarily by crackers and warez d00dz, for which reason hackers use it only with heavy irony. The term used to refer to the folks allowed in to the "hidden" or "privileged" sections of BBSes in the early 1980s (which, typically, contained pirated software). Frequently, early boards would only let you post, or even see, a certain subset of the sections (or `boards') on a BBS. Those who got to the frequently legendary `triple super secret' boards were elite. Misspellings of this term in warez d00dz style abound; the forms `l337' `eleet', and `31337' (among others) have been sighted.
A true hacker would be more likely to use `wizardly'. Oppose lamer.