IYBY is a variant of the "that's what she said" response. It has multiple applications. You can also alter and utilize portions of the phrase: "inside you", or, when shortened "in you".
A trip to a grocery store, convenience store, or gas station for the sole purpose of buying an excessive amount of junk food and sugary drinks, usually with friends. A chud run often happens late at night and ends with everyone involved hating themselves afterwards.