The origins of "AA制"

Question

Every since I learned the expression AA制*, I have been curious as to its origins. The morphology seems clear enough:

AA + 制(定) = "to draw up (the bill) in an AA way".

What is not clear is how AA acquired the meaning of "evenly" or something like that. The use of roman letters implies it's borrowed from another language (possibly English). Some Chinese netizens have suggested exactly that. However, I can't take seriously their suggestion that AA is an abbreviation for "Acting Appointment" or "All Apart". I know that I personally have never heard either of those expressions uttered by an English speaker.

So, any ideas where AA制 came from?

_{* For those of you who don't know, AA制 means to split the bill evenly when eating out. Or as we say in English, "go Dutch" (apologies to the Dutch).}

Answer 1

When I lived in Guangzhou I was told the expression came from Hong Kong and stood for "Algebraic Average".

That also doesn't sound like something a native English speaker would come up with, however I don't think that's a reason to discount it (or even the other suggestions) if it came from Hong Kong where non-native English speakers come up with all sorts of part-English phrases that sound awkward to us but still become popular, like "他真没有sense" to mean "he has no taste" being one I heard recently.

Then again I can't be sure if "Algebraic Average" is the true etymology either, just offering my two cents.

Answer 2

Wikipedia has this to say:

• 「AA」是「Arithmetic Average」的縮寫。意思是「算術平均」。
• 「AA」是「All Average」的縮寫。意思是「全部平均分擔」。
• 「AA」是英文「Acting Appointment」的縮寫。

There is no citation or whatsoever, so these could all be wrong...

百度 also has something to say. Despite the total lack of structure, there is an interesting excerpt from the article:

Chamber's English Dictionary的解释，AA，aa或者a.a.的意思是：(in prescriptions, etc) in equal quantities of each). Origin: Late Latin "ana", from German, again。意即：（在处方中）各以等量。来自拉丁文ana，起源于德语，“再次”。

This fits quite well, but then it leaves me wondering why a remote medical term would become popular in a different sense..