Maybe the original author was thinking "a" as in alpha rather than "a" as array and "c" as in character? Those prefixes get kind'a tricky sometimes, especially if you switch between different languages. I tend to use "b" for boolean rather than "l" for logical, since "l" for me tends to be long.

Must be close to lunchtime -- I just noticed that a piece of code a moment ago has an odd variable name like cSiu ("char siu" -- i.e. Chinese barbecue pork).