When you've totally consumed a carbonated beverage of some sort, or any food that creates gaseous fumes out of the mouth and you need to express it fully (meaning, you can't keep your mouth closed or somehow swallow the burp because you know your neck will explode). And while you express it fully, you say while burping, "Botch." Some others, continue saying the entire word even if the belch has ceased. Usually, after a good botchulism, men receive erections on the spot. Women usually have a quim-like experience.
"Dude, Amy had a nice botchulism last night. It was so loud, I fuckin' got a huge boner."