When your touchscreen phone emails someone while in your pocket. The message usually contains a lot of random letters. A slightly less awkward (but still awkward) version of a buttdial.
Oh no, my phone came unlocked in my pocket and buttmailed my boss!
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.