idrm is a misspelling of, "idrk" which means "I don't really know." This misspelling came into play because the letters 'k' and 'm' are close to each other on most keyboards, making is irresistible for most fools to make the mistake.
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.