The word "sick" is used primarily
by teenagers and wannabees to mean "cool" or "fresh".
Sometimes is is said in a drawn out form for emphasis such as "s-i-i-i-i-i-i-i-ick".
It can also be used
"coolness" when someone falls
or attempts to do something
amazing and sucks instead.
A sick whistle is the act of saying the work "sick" twice
in a row very quickly in a male falsetto voice.