Sorry if this is a completely stupid question (I'm a not a settheorist, though I've been doing some reading in the subject), but I was wondering, specifically, about the exact provenance of the name. I know, of course, that it's after Woodin himself, but who coined it? Where is its first appearance in the literature? Just curious.

The notions of Shelah cardinals and Woodin cardinals were introduced by Shelah and Woodin in their joint paper
which itself was the result of the hugely influential Martin's Maximum paper by Foreman, Magidor, and Shelah. In their "Lebesgue measurable" paper, Shelah cardinals are those $\lambda$ that satisfy property $\mathrm{Pr}_a(\lambda)$, and Woodin cardinals are those that satisfy $\mathrm{Pr}_b(\lambda)$. Overall, the current notation seems better. To get an idea of how quickly the term was adopted, already the ShelahWoodin paper mentions it, see page 384 and Definition 4.1 in page 392:
Note that, in spite of its publication date, the results of the paper were obtained quickly after the results in the Martin's Maximum paper, itself published in 1988. In the $\mathsf{MM}$ paper, we read (page 27)
Woodin's result indicates how to prove from large cardinals that $L(\mathbb R)$ is (elementarily equivalent) to the $L(\mathbb R)$ of a Solovay's model. Working on optimizing this result led to the notions of Shelah and Woodin cardinals. The relevance of Woodiness was quickly seen in the context of homogeneously Suslin sets, relevant to determinacy, which led to the immediate adoption of the notion. The first appearance of the term in the literature is in the papers by Donald A. Martin and John R. Steel,
and
Both terms "Woodin cardinals" and "Shelah cardinals" are probably due to them, but due to the influence of the concept, the terms were in use, particularly Woodin cardinals, before the papers appeared. For somewhat more precise dates, I suggest looking at
Steel mentions 1984 for the isolation of the concept of Woodinness, and 1985 for the proof of projective determinacy (see column 1 in page 1147). 

