Adam contends that a "slider" is not a mini hamburger but, instead, the following:
"People, a slider is something very specific. It is not just a mini hamburger. It's a thin, thin slip of beef, cooked on a griddle with onions and pickles piled atop patty. The steam from the onions does as much cooking as the griddle. The buns are placed atop the onions, absorbing the pungent aroma and flavor."
I was incredibly interested by this because it's so precise and George Motz further perpetuates the same in his book, Hamburger America. This spawned other burger lovers to take up the same definition.
David Gerard Hogan claims that White Castle's burgers have been called "sliders" since the '30s but no such evidence exists with the earliest record being in the '80s (Barry Popnik).
I refer to my man Barry Popik for this next bit:
"There is considerable evidence that "slider" was a term used for a hamburger in the United States Navy, perhaps as early as the 1940s or 1950s. The term "slider" meant a greasy burger that slid in easily."
Nowhere in there does it say it has to be cooked on a griddle or have onions. Merely it's a term that was used by the NAVY to refer to greasy burgers that essentially "slid." This is the earliest reference to hamburgers being called sliders that Barry has found. I couldn't find anything else myself.
I was further intrigued, though. When was the first time the word slider was used in reference to food? Oxford English Dictionary loves me so and I came up with this:
Ice-cream served in a sandwich form between two wafers. colloq.
1915 J. J. Bell Wee Macgreegor Enlists ii, Come on oot wi' me an' I'll stan' ye a dizzen sliders
That's before White Castle even existed. If anything, a slider is an ice cream sandwich!