Yeah, no. Let's not pull the pin on that hand grenade.

The translation of explatives into Dovahzul has a long and bloody history, but the short answer to your question is that for the forseeable future, there is not, and will never be, a Dovahzul word for "fuck" in any kind of literal sense. As an interjection of frustration (ie. oh, fuck!), we like to use the word "ruth," meaning rage.

If you're talking about fornication, well... get creative. I've got no suggestions for that.

There is no canon word for "fuck"; however the non-canon word is, I believe, "gjok". 

I do not use these types of words when I'm trying to offically translate an English text into Dovahzul, but it sure is fun to use in real life, or online when angry at noobs. You'll find that even EA does not censor non-canon Dovahzul swear words.

My other favorites (all non-canon, I believe) are Draaf (shit) and Vik (damn). Ugh, I can never get enough of loudly yelling these at school when I drop something or if a teacher assigns too much homework.