Origins of the Foxish language
Littlefox was an unbearably cute fox character played by Foxen, ca. 1990 on FurryMUCK, who spoke in funny, unbearably cute noises. Foxen created a list of these sounds, with an optional modifier, and these were adopted by some of the fox characters on the MUCK.
The word yiff was originally simply a pleasant greeting sound, and someone who was happy might say they were yiffy. The term became corrupted from its original meaning, and ended up coming to imply being horny, thus evolving yiff to its current, most known meaning, that of one related to sexual activities.
There are seven main sounds in Foxish, plus an additional two "slang" sounds. The basic sounds are on a continuum from positive, happy, affirmative meanings to negative, unhappy, or disagreement meanings.
|growf||No, unhappy, negative|
|growlf||Very negative, unhappiest|
|murph (slang)||Contentment|
|yaff||A neutral (slightly positive) sound|
|yarf||A neutral (slightly negative) sound|
|yerf||Happy, positive, a greeting|
|yiff||Originally: Happiest, very positive, yes, an exuberant greeting; Now: sex|
|yip||Happier, more positive|
|yipp (slang)||Sexual proposition|
The modifier -le, when applied to these sounds, softens the meaning of the sound and makes it less emphatic. For example, yerfle can be a friendly but more sedate greeting than yerf.
As you can see, this is a very limited language, used primarily for expressing emotional states. This is not a bug, but a feepture, as littlefox, the original speaker of Foxish, was a very simple (but also very cute) fox.
Uses of Foxish
- The name of the art archive site Yerf is a Foxish word.
- The YARF! fanzine pre-dates the invention of the Foxish language, and the word was borrowed for the lexicon.
- Real foxes don't really sound much like this, but according to Foxen, the sounds "were cute sounding at the time [and] that was reason enough." 
- The name of the now-defunct furry 'links directory' Yiffle is a softened Foxish word. The website described it as "the sound of a happy fox".