We know from the Guide that the general term comes from "Sha'je duels, held at Qual, [which] involved the use of left- and right-hand dag­gers, called respectively osan'gar and aran'gar, tipped with slow poison."

Thus, those names were certainly chosen to indicate that the second chance Forsaken are supposed to be deadly tools for the DO to infiltrate the Lightside as unseen threats.

However, I wonder if the literal meaning of aran'gar is really right-hand dagger, as the Guide indicates. The word "aran" also appears in tel'aran'rhiod, which probably either means World of Dreams or the Unseen World. So, maybe aran'gar rather means something like Knife/Dagger of Dreams (which would be a nice allusion to the title of the book), or something like "Unseen Dagger" (which might point to Perrin's "unseen foe" in ToM). One wonders then, though, what osan'gar means.

On the other hand, the "haran" in Shaidar Haran, apparently means "Hand of", so if "aran" means right-hand it also be related to this (since SH is basically the DO's right-hand). Maybe tel'aran'rhiod literally means then something like the "not-right-world", in the sense of an unreal world?