I work at a technology company, and we frequently use the term 'rathole' to describe a topic (generally one brought up in a meeting) that's a tangent. And when I say I we use it frequently, I mean the term came up five times today, including in the verb form: "I don't mean to rathole here, but [insert marginally germane discussion here]...." So it was a surprise to me that this term is not particularly well-known.
Ratholing is a term used to describe a conversation or process that has deviated from its original productive purpose into a generally unproductive but long and winding detour that eventually comes to a dead end. The original discussion purpose may be to agree on a course of action. However, if one or more people rathole into a specific point of the discussion then the discussion stalls with no actionable outcome. This term is frequently used on the Macbreak Weekly podcast, resulting in the eventual creation of the “Rathole!” jingle and subsequent full-length song.
Terminology note: "RAT" is also the term for the Rock Abrasion Tool on the Mars rovers, which can dig holes in things. Makes for some interesting Google Image searching. See? That was kind of a rathole, just now.
So what about you -- do you rathole? Or do you "take it offline?" (Blech.)