[Lord of Angmar]

I can recall '*** ' being used twice in the Lord of the Rings, but never in a crude manner as it would be used today

*** ! Fool! Thrice worthy and beloved Barliman! (Gandalf at Bree)

There was also a line something to the effect of "Sam, you old *** !" said lovingly, not despararingly, by Frodo (I believe in Ithilien). I agree with the initial post in that burarum is probably the closest to a curse uttered in any Tolkien-created tongue of Middle Earth, and with the Barrow Wight in that if anyone were to truly curse in Middle Earth, it would be the orcs and servants of the Enemy.