1. A place where anything is possible but nothing of interest is practical. Alan Turing helped lay the foundations of computer science by showing that all machines and languages capable of expressing a certain very primitive set of operations are logically equivalent in the kinds of computations they can carry out, and in principle have capabilities that differ only in speed from those of the most powerful and elegantly designed computers. However, no machine or language exactly matching Turing's primitive set has ever been built (other than possibly as a classroom exercise), because it would be horribly slow and far too painful to use. A 'Turing tar-pit' is any computer language or other tool that shares this property. That is, it's theoretically universal -- but in practice, the harder you struggle to get any real work done, the deeper its inadequacies suck you in. Compare bondage-and-discipline language.
2. The perennial holy wars over whether language A or B is the "most powerful".
From The New Hacker's Dictionary, edited by Eric Raymond. (The MIT Press)