The Ernesto is the act of fucking a chick with a condom on, and then at the last minute, right before you come, without warning, pulling the condom off. The goal is to cum all over the girl and make as big a mess as is possible, much to her surprise. It is not an Ernesto unless it is an unwanted surprise for the girl. It makes little sense and is completely inconsiderate, which make the thoughtless act quite entertaining. It is so named after being pioneered by an Italian man named Ernesto.
"dude, I was about to cum and I figured "fuck it, I don't even like this chick", so I had to Ernesto her at the last minute"