One of the tricks in the stable of the midwestern playground bully. To snicker-snag someone is to hold them down, dangle a thread of spit out of your mouth and while the spit is dangling, dip your head closer and closer to the victim. threatening to drape the spit on their face. The spit does not reach its target in a true snicker-snag, but is instead, sucked back in at the last possible second. It is a variation on snagging, which is to actually spit on someone. The snicker-snag (and snagging in general) was brought into the popular vernacular by Mystery Science Theater 3000.
"Death by snicker-snag!" - MST3K's Mike Nelson, during a scene in which two guys are fighting and one is holding the other down.
"I want to kick this movie in the groin and snag on it!" - MST3K's Crow, during Starfighters