I think you are right !!! Taboo seems the closest-word to define "Khalum", the other term for me would be "inappropriate".
The elders use that word sometimes to teach young boys/girls about good manner (sometimes they are too lazy to explain, so they use this shortcut word), even when there is no logical explanation for you. Khalum is not necessary linked to superstition stuff !!
For instance: the elders used to say for girls(ancien time) : "Don't wash your hair(shampoo) at late night" --> They don't want you to get cold & fall sick.
Another phrase that comes to my mind now is: "Ya su nonn kin, man khalum ! " --> for the good digestion it's better to sit & eat correctly
So, the next time that your parents use this shortcut taboo word: ask them "WHY ?" lol.
you will be surprised by the reason lol...