When during a sloppy blowjob the blower gags twice then vomits profusely, whilst tossing their head back, either causing a mess or an orgasam. The 4 Billion referring to how many nightmares the blowee will be having.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.