When the world ends and it's just you and your bro left in planet Earth you agree to have butt sex as long as one bro doesn't cum in the other bro's butt. Once there is insemination trust butt is over.
Sean broke the sacred code of trust butt by cumming in Brandon's ass.
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.