"around" 

This article is nicely written, since it details the historical use, and not just the word's origins and definition.  Circa literally means "around" in American English; I remember vaguely that in British English "about" carries the same concept of following a curve or circle.  I added "around" since "about" isn't often used in this sense by Americans.  Also, there could be links to circumference, circumvention, and other cognates.