WordNet (r) 3.0 (2006):

rathole
    n 1: a hole (as in the wall of a building) made by rats
    2: a small dirty uncomfortable room

The Jargon File (version 4.4.7, 29 Dec 2003):

rathole


    [from the English idiom ?down a rathole? for a waste of money or time] A
    technical subject that is known to be able to absorb infinite amounts of
    discussion time without more than an infinitesimal probability of arrival
    at a conclusion or consensus. ?That's a rathole? (or just ?Rathole!?) is
    considered a pre-emptive bid to change the subject. The difference between
    ratholes and religious issues is that a holy war cannot be pre-empted in
    this way. Canonical examples are XML namespaces and open-source licensing.