I dunno when it is started, but this is my favourite greeting words...
i used it everytime i meet my friends and it'll come out naturally... try meet me once and u'll got to hear it. heheh

the Yoosshh word give me some kind of happy and energetic feeling thus creating pleasant experience of meeting peoples. U can try it yourself if u want. i think its better than other greeting words like hello or good morning etc. hehe.
try this :
" Yoosshh! whats up?"

I used to watch japanese series and movies a lot and maybe thats where the word come from. The japanese people are well known as hardworking and high spirited people. So, often when they are about to start off their work, they'll shout "Yoosshhaa..!!!" loudly as it give them strengh and boost their spirit.

Be it for greeting, or boosting spirit, the yoosshh words really gets up your mood. So, thats about my style of greeting... what's yours? heheh