Interesting question, it drove me to google. Amazing what you can get if you just pose a question to google like "Where does the term dry hopping come from?"
Here's a link to a discussion I found:
https://homebrew.stackexchange.com/questions/14526/dry-hop-etymology
The people in the discussion indicate no clear reason why, though there is some speculation that "dry" has to do with pellets as compared to "wet" hop cones, or "dry" as an antonym to "sweet."
Personally (this is me with no evidence other than...well, me) I think this last one makes some sense. At the start, wort is sweet, and doesn't have a "dry" finish. After fermentation, the beer has dried out in finish, so hopping would be done when the beer is "dry." But this is just me trying to make sense of it, I'm no authority on etymology or dry hopping.
Here's another:
https://www.hopculture.com/what-is-dry-hopping-craft-beer-brewing/
The article indicates "The term dry hopping comes from the fact that the hops are placed dry into the fermented ale. Though they do get wet in the process, they are not boiled in the wort, so the name stuck."
This makes less sense to me. The hops used during the boil are placed dry into the boil, so I don't see the difference.
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Jonny, the above (go to google) was not meant as criticism. It's a beer group, good topic for discussion. Whereas a single query to google might satisfy one's curiosity, no one else benefits from it. Posting here means lots of people get to learn about it, and if they don't want to click the link for the thread, they don't have to.
I've found myself using google more and more lately to look things up. Electronic dictionary, don't you know. Ran across some words I'd never seen before. Such as sockdolager. Look it up.
