I think that it does have something to do with Numenorean. I don't know if tark in orkish slang would be used as a swear word, but its real meaning is this:
Quote:
|
In this jargon tark, 'man of Gondor', was a debased form of tarkil, a Quenya word used in Westron for one of Numenorean descent.
|
-Appendix F, LotR
If we put that with what Snaga said
Quote:
|
He's got past the Watchers, and that's tark's work.
|
-RotK, The Tower of Cirith Ungol
we can see that the Watchers have some kind of weakness to those of Numenorean blood.