"

 ""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.  "