Hunky–So, don’t ask why I looked up this word; I couldn’t tell you. However, it’s not used in Regency. The only reference I could find to it was in an 1810 burlesque play called Tom Thumb the Great by Kane O’Hara. There’s a character named “Princess Huncamunca,” and at one point, another character says to her, “Come my Hunky, come my pet.” I’m not sure whether I’d prefer the nickname or “Huncamunca.” So, if you happen to be writing about a Regency character called “Huncamunca,” feel free to use the diminutive, “Hunky.” Otherwise, you’re out of luck.
Water(ed) down–This one involved me having to slosh though a lot of treatises about proper well maintenance in the 19th century. I deserve a gold star for getting through it. Now, according to Merriam-Webster, the expression, which means to reduce the effectiveness of something, wasn’t used until 1850. Oh contraire. I found evidence of this being used in 1811 in a rather tongue-in-cheek letter to the editor of the Christian Observer about the hyperbole used by the author’s sisters:
I can pretty well judge how far the strength of the expression is suited to the subject; and water down as I see occasion.– But, Sir, it is on their account I write to you; upon them the effects of all this are very severe. In the first place, when any really great occasions arise, they are sore distressed indeed. Having used up all the strong words of the language upon weaker topics, as they cannot swear, they are obliged to be silent; and having expended their strong emotions in the same prodigal manner, they have no resource but an hysteric.
The writer goes on to ask of his sisters to stop calling him a “little stiff Quaker”–the real reason for his letter saved for last.
As this book was printed in London originally, I think the expression would be safe to use as an English expression from 1811 forward. Would it have been used by the upper class? That part is unclear to me. Use with caution.