I would have thought that being able to understand what your wife is saying to, or about you is definitely essential to any relationship.Why was it locked

After consulting with 3 different Thais and a farang, my understanding of the expression, which I would readily admit is less than complete, is that it is NOT POLITE, and is something of a dismissive, throw away expression, used where you don't agree with someone and that you don't like, don't care about, or have any interest in them (maybe because of the disagreement?).

All are quick to say it is not polite.

Will keep my ears tuned for that one!

I've been told that it roughly translates to English as:
"serves you right or you got what you deserve or you should have known better or karma, etc.".

For example:
You get arrested for driving while intoxicated, som nam na.
You're stupid enough to get caught cheating on your wife (coming home stinking of cheap perfume & lipstick on your collar), som nam na.
A student who doesn't study & fails the class, som nam na.

I've never heard it to be impolite. Maybe, unless you say it to the person experiencing "som nam na".