Why Do Some People Get Called 'Ah Sir' in Hong Kong?
In fact, there are specific parameters which govern when someone’s an “ah sir” and when not.

The Cantonese phrase “ah sir” (or 阿Sir) is a uniquely Hong Kong form of address, a term of weirdly colloquial respect.
On the face of it, it’s a straightforward loan word from English. Just like the dik si, see dor and baa si—taxi, store and bus—“ah sir” comes straight from English, the word 阿 indicating you’re naming someone and “sir” being the honorific term. It’s also more rarely written ah seh (阿蛇), essentially—“Mr. Snake.”
But there’s a little more to it than that. Any old knight isn’t just “ah sir.” In fact, there are specific parameters which govern when someone’s an “ah sir” and when not. Most often, it breaks down into two simple categories: policemen and teachers.
All (male) teachers are “ah sir” as a matter of respect. (Female teachers tend to be “miss,” pronounced “miss-see”) If you wish to refer to a teacher by name, then it’s last-name-sir: So if Mr. Chan teaches you calculus, then to you he is Chan Sir. This applies to all teachers—unless, of course, you’re sufficiently famous or good at branding. Take super tutor Ken Ng, founder of tutorial center Modern Education—he’s known throughout Hong Kong not as “Ng Sir,” but as “Ken Sir.”
For coppers, it’s slightly different. If you, a member of the public, needed the help of a policeman on the street, you’d go ahead and call him “ah sir”—or you’d be wise to, if you wanted his assistance. These cops in turn would reserve their “ah sirs” for their ranking officers. An example is Chief Superintendent Steve Hui Chun-tak, who made televised briefings every day at 4pm during the Occupy protests. The public liked his candor—and a few of his catchphrases—and so he became universally known as “Four o’Clock Hui Sir.”
It can get more complicated than that, though. Take 2011 TVB series “Yes Sir. Sorry, Sir!” The plot concerns an ex-teacher who becomes a cop—and is then sent undercover back into the school he used to teach in. In Chinese, the show is called “點解阿Sir係阿Sir,” or “Why is ah sir ah sir?” A pun? An existential question? Or both?