The 1980s mid-1980s
Leg Warmers
Knitted or stretch-fabric tubes pulled up over the calf, borrowed wholesale from ballet studios - worn scrunched down over tights and sneakers by people who had never taken a dance class.
Leg warmers exist, originally, for an entirely practical reason: dancers wear them backstage and during warm-up to keep calf and ankle muscles warm and loose between rehearsal segments, then peel them off before performing. There's no fashion intent whatsoever in the original garment - it's a knitted or ribbed stretch-fabric tube, open at both ends, pulled up over the lower leg purely to prevent muscle strain in a cold studio. That's a genuinely unglamorous origin for a garment that ends up as one of this decade's most immediately recognizable visual shorthand items.
The jump from backstage necessity to street fashion runs almost entirely through the aerobics boom and the dance-studio aesthetic Flashdance and Fame both put in front of a mass audience early in the decade. Once leg warmers read as "dancer" or "serious about fitness" rather than merely "backstage necessity," people who had never set foot in a studio started wearing them over jeans, tights, or bare legs, scrunched down deliberately rather than pulled taut - a styling choice that has no functional justification at all, purely a visual one borrowed from the shape they happened to fall into after actual dance use.
I think leg warmers are a genuinely clean case study in how this decade's aerobics silhouette worked more broadly - taking an item with a real, specific, physical justification in one context and importing it wholesale into everyday dress purely for what it visually communicated, with the original function stripped out entirely. Nobody wearing leg warmers to the mall in 1985 needed warm calf muscles; they needed to look like someone who might.