what came first: the word elastic or the item of elastic?

Factual Questions
1

I think the OP says it all.

2

Obviously elastic items existed prior to human language, in the form of natural rubber. But assuming you mean when did we refer to items as elastic, the noun usage (1847) came after the adjective (1674), according to Merriam Webster.

3

The word. It’s a scientific/engineering term that was applied to materials long before the mass marketing of the elastic band.

From here:

elastic (adj.)
1653, coined in Fr. (1651) as a scientific term to describe gases, from Gk. elastos “ductile, flexible,” related to elaunein “to strike, beat out,” of uncertain origin. Applied to solids from 1674. The noun, “cord or string woven with rubber,” is 1847, Amer.Eng.