The Meaning of "Check-Six"
The term "Check-Six" has its origin in common direction-telling methods (as seen in films like Indiana Jones and the Last Crusade, or Twelve O'clock High). For example, "I see an airplane at 11 O'clock"... referring to the bogey (another aircraft) being to the front and slightly left of the aircraft ("12 O'clock" being directly ahead). Using this technique, "6 O'clock" is directly to the rear of your aircraft (and is also a very vulnerable position to be attacked from), or otherwise your posterior. Thus, the term, "Check your Six O'Clock," or as shorten for brevity-sake to "Check-Six," means to watch your rear.
For us, it also refers to looking back into aviation's past.