What did writers mean by "plastic" in the late 19th century?

Flexible

I guess from the word “plasticity”?

The “y” word probably came second. Think rubbery or mighty, or flexibility.

Could it also mean fake?

Plastic has become a stand in for cheap. But plastic itself as a modifier usually implies bendable or malleable. Think about the word plastic in front of ethics or character.

Can also mean “formative/creative”. That’s a bit archaic, though.

History and Future of Plastics 1869 plastic billiard balls.

The plastic arts are those having to do with sculpture and modelling..

moldable

My boss used to use that word a lot. He meant it to mean stiff and unbending. He wasn’t an artist, so it might mean something completely different in the art world

“plastic… I. a. Having the power of molding or shaping formless or yielding material (as, “that divine plastic force which is for ever molding human society”: Mrs. H. Ward’s “Robert Elsmere,” xxxii.) concerned with or pertaining to molding or modeling (as, plastic arts, such as sculpture and ceramics); produced by molding (as, plastic figures); also, capable of being molded or of receiving form (as, plastic substances, such as clay, wax, or plaster); fif., capable of being brought to a definite condition or character (as, “The world is plastic for men to do what they will with it,” H. G. Wells’s “Mr. Britling,” ii. 1~6; the plastic mind of youth); pliable; impressionable; in surg., concerned with or pertaining to the remedying or restoring of malformed, injured, or lost parts (as, plastic surgery; a plastic operation); in biol. and pathol., concerned with or pertaining to the formation of new tissue in the living body. II. n. A plastic substance. ... The quality or property of being plastic; esp., capability of being molded, receiving shape, or being brought to a definite form or character.”
-from The New Century Dictionary, 1944 ed.