Pwn or pwned comes from the verb to own or meaning to conquer or gain ownership. In the case of being pwned online, it typically means the hacker has your credentials to a Web site that has been hacked at some point.
What does it mean if I've been pwned?
Being pwned means that a hacker has your credentials to a Web site that you've once subscribed to. For example, Adobe: In October 2013, 153 million Adobe accounts were breached with each containing an internal ID, username, email, encrypted password and a password hint in plain text. Compromised data: Email addresses, Password hints, Passwords, Usernames.