Once upon a time, before your best girlfriend wore a shirt as a dress and you looked at your post-sex hair and were like, yeah, that works, hot mess described a meal.
In the 1800s, it was used commonly as a soldier's meal rations. It could be from the Latin missus, which is a portion of food. Maybe a hot mess was a soup or a gruel. It is unknowable what magic made a mess into a hot mess, but that's the current curiosity surrounding messes and hot messes today, so isn't that deeply fascinating? We change in so many ways, but truly remain the same. We are a remarkable, word-speaking people.