A slang term used when telling someone to calm down and relax. It is pronounced “woo-sah,” with the “oo” of “toot” and the “sa” of “saw.” Woosa. This is an example of an appropriate usage of woosa:

-“OMFG! What if Billy doesn’t like me?! My life will be ruined!!!”
-“Woosa, bitch.”

It can be self directed. If you're stressed, just say woosa. For added affect, it is customary to grab your ear loops between the thumb and forefinger of each hand and rub in a gentle circular motion.
Like most slang terms, especially those that originated as exclusively spoken words, there’s no official spelling for woosa. You might see it spelled as “woosah” or something similar. “Woosa” appears to be the most accepted spelling based on a head-to-head Google search competition.

The origins of woosa are unclear, but the word hit mainstream pop culture in 2003 with its frequent use in the movies Bad Boys II and Anger Management. It’s a very contagious word, you might find yourself using it without realizing it.