A place where anything is possible but nothing of interest is practical. Alan M. 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.
A tar pit is a geological occurence where subterranean tar leaks to the surface, creating a large puddle (or pit) of tar. Animals wandering or falling in get stuck, being unable to extricate themselves from the tar. La Brea, California, has a museum built around the fossilized remains of mammals and birds found in such a tar pit.