First, a short word history: the idea of "Fwching!" was originally conceived while reading the engaging, clever comic known as Calvin and Hobbes. Calvin was depicted as shooting something with a small twig-constructed, y-shaped slingshot. The onomatopoeia used was "Fchwing!". I, the reader, liked the word very much and decided to add it to my vocabulary (but, of course, never actually say it aloud). However, while typing it on one occasion, I misspelled it as "Fwching!". This corruption of the word was the basis for the all-purpose word that is known and loved today (by me... just me).
1. Boom!, crash!, pop!, etc. (onomatopoeia)
2. Wow! (to express excitement or emotion)
3. Quite simply a word used when one has nothing else to say, or DOES actually have something to say (which would be "Fwching!"). See supercalifragilisticexpialidocious.