Will some one define pessimistic for me

A pessimistic is a person who tends to see the worst aspect of things or believe that the worst will happen.

Let me start off with this:
this is what google says:
tending to see the worst aspect of things or believe that the worst will happen.
and now, let me explain.
So, basically it is when you think the worst you can think will happen, because as you can see, that it says :
 see the worst aspect of things or believe that the worst will happen.
that really tells clearly, that yes, that is when you start thinking of something bad that will happen.
Hope that this helps you! =)