Internet slang, used when you think something intended to be funny is not. The "zzzz" represents falling asleep. This word looks similar to "lolz", meaning multiple laughs. However, the definitions are opposite.
This word could be used with as many "Z"s as necessary, depending how lame the joke/story is.
Girl: Hey, why did the chicken cross the road? To get to the other side! Hahaha!