A chump is a sucker or a fool — someone who is very gullible. If a con artist cheats you out of money, you may end up feeling like a chump.
Chump is a very informal word for someone who falls for every trick and scheme, or who believes everything you tell them. If you get an email from an African prince asking for your bank account information so he can give you a lot of money, and you respond excitedly, ready to collect the cash, you're a chump. This meaning of chump first appeared in the 1880s — earlier, the word meant "short, thick piece of wood."