“Plastic surgeon”… Is that a surgeon made of plastic? Or maybe a surgeon specializing in operating on latex dolls? (George Costanza of “Seinfeld” fame and his stint as latex salesman at Vandelay Industries come to mind, or to those Scandinavian, the history’s probably one and only hit song dedicated to an inflatable doll. It was 1969 and it was Swedish) :-).
Of course we know (somehow) that a plastic surgeon in English is also known as a cosmetic surgeon and is a live person operating (and funny enough: NOT “surging”, similar word, seemingly with the same root, but with a quite different meaning) on live people.
Why the confusion, then? It is because in this context “plastic” is an adjective. It’s derived from the Greek word ‘plastikos’ meaning to mold or shape. It so happens that it’s the same word in English as the noun “Plastic”, which refers to usually stiff objects made out of a plastic (<–adjective) material, a material to which we often refer to as simply “Plastic” (<–noun). And voilá: an easy joke.
A deficiency (or difficulty) of English is that often a noun and an adjective version of a word is the same word: “bet on the red (<–noun)” and “the red (<–adjective) bike”. And that then leads to another issue with the English language: since the two forms of a word are often the same, some times a different (correct) adjective form exists, but is not used in daily speech. Case in point: “Face surgeon”. Yes, we all know it’s the doctor operating on your face. But since “face” here is supposed to be an adjective describing the surgeon, we are actually here strictly speaking describing a surgeon facing us (in a crowd). Like in “face value” (the value you can see). The correct wording is actually “Facial surgeon”, because “Facial” is the adjective pertaining to what happens to a face.
But nobody talks like that.
Linguists, however, sometimes have to understand nuances between how a language should be used and how it is actually used, because otherwise it can come out really funny at the other end.