What is the origin of SLACKER ?

Gee, I never thought it could be anything else but a reference to something that was slack, say a rope.   A slack rope doesn't hold up anything, you see; a tent cord, a stay or shroud, a guy wire: if it's slack it isn't doing its job.

As for using it without hurting someone's feelings, for that you need not linguistic instruction but moral.   In my experience, when I'm annoyed over someone's fault there is no way to accuse her of it without communicating my annoyance.   There can certainly be times when it is more urgent to make the communication, even accompanied by my anger, than to wait until I can do it better; but in most cases I regard it as my job to deal with my annoyance first (taking myself by the scruff of the neck if necessary) and only then talk to the slacker.   More often I discover, once I'm no longer annoyed about it, that it's better not to say anything at all.