See yiff. Although yff is often seen as a typo for "yiff", the occurence of the "yff" spelling is so common that it is feasable to think of both of these spellings as correct. If using this word primarily in it's otomatopoeia form, it seems that "yff" is more commonly used, but when using this word as a subject or part of a sentance, "yiff" seems to be more proper.
