(Noun) An open-faced single condiment sandwich. Usually a delivery vehicle for that condiment, which would be impractical to eat any other way. Key is that it remain open faced, with a single ingredient (aside from the bread).
Most Common: the "Mayonnaise Big", but also a "Mustard Big", "Miracle Whip Big", "Ketchup Big" or even a "Relish Big".
Origins: PNW, Southern Oregon in the mid to late 1970's, but may date from even earlier.
I was looking for a midnight snack, and the refrigerator was nearly empty, so I had a Mayonnaise Big.