Some friends were hotboxing a car, having a coherent discussion about a topic that is no longer remembered, when one of the friends remarked, "...a spontastic lightning strike."
The root of the word is a bit of a mystery, but most likely it is a stoned mixup between spontaneous and fantastic. Such can be taken as the meaning.
Five minutes after the session, I was spontastically stoned.
I had some spontastic sex last night...with your mom.