1. when something is so thorny (aka awkward) that it is pleasantly entertaining (usually for a third party)
2. when someone is so overwhelmingly thorned that he or she breaks out in nervous, sometimes uncontrollable laughter
1. The character "Murray" on Flight of the Conchords provides plenty of thorngasms.
Guy: "My girlfriend farted in the car on the way home and I pretended like I didn't hear, but we could both smell it."
Friend: "HAHAHA that's a thorngasm."
2. The doctor asked me if I was sexually active in front of my mom and I had to answer yes. He then proceeded to ask me all storts of questions about my sex life. My mom just stared at me in disbelief the whole time. I almost had a thorngasm.