A waddy is a piece of shit with corn (or peas) in it.
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 1: "Oh man I just did a
big waddy.."
Person 2: "oh
yuck dude, was there much corn?"
OR
"your'e a big
fat waddy"
"your'e a piece of shit with corn in it, you waddy!"