A Fantasmagasm happens when something is really, really awesome. Not necessarily sexual. It consists of an immediate smile and jumping up and down in giddy fashion going "Omg! That's so awesome!" or something to that effect. It is pure happiness and excitement.
A word used to descibe a fantastic sexual encounter or continuous sexual encounters in which both or all parties benefit by receiving one or more orgasms; Incredible sex which both or all partners find mutually rewarding in a purely physical capacity
It's the word "Fantastic" with the word "orgasm" in the middle. The word describes something that is so fantastic that it makes you orgasm while you are doing it.