Sometimes when you don't chew peas or corn enough, they become viewable when you do a shit. When you call someone a waddy, you are calling them the above description. A waddy looks very discusting and sometimes dogs do them too. So when you eat corn and notice sweetcorn in your shit, you just did a waddy.
Person 2: "oh yuck dude, was there much corn?"
"your'e a big fat waddy"
"your'e a piece of shit with corn in it, you waddy!"
Him(over IMing later that night) I'm sorry I have been ignoring you. I pulled a waddy. im sorry. i will never waddy you again. I love you.