Green’s Dictionary of Slang

1. an ice-cream placed between two wafers.

1995

Sean Power Those Were the Days n.p.: On presenting our thrupenny bit, we received what we called a slider, or wafer [BS].

2. (US) a small, greasy hamburger [poss. orig. navy slang].

1984		LaBarge & Holt Sweetwater Gunslinger 201 (1990) 192: He could already taste the slider and autodog on Hambone’s meal ticket.
1999		J. Lethem Motherless Brooklyn (2000) 2: Jaw working to grind the slider into swallowable chunks.
2004		(ref. to 1960s) on ADS-L 🌐 The slang term ‘slider’ meaning ‘[small greasy] hamburger’ [...] was used in the 1960’s according to my own recollection. It was not restricted to White Castle in my experience. The folk etymology said that the burger was small and greasy enough that it slid down the throat without the need for active chewing or swallowing ... or something like that.
2004		(ref. to mid-1940s) on ADS-L 🌐 He recalls them being called sliders back then [i.e. the mid-1940s]. And, in his opinion, the reason the little burgers were called such was because they went in – and out – shall we say, without stopping.
2017		T. Pluck Bad Boy Boogie [ebook] Denny got three [hamburgewrs]. TThey looked like White Castle sliders in his hands.

3. (UK prison) a hacksaw.

1954

J. Phelan Tramp at Anchor 179: Tike me davy that bleeder ’ad a comb an’ slider in ’is flowery.