One of the things that struck me is that many readers of this blog seem to have found their way to us as a result of searching the internet for the word thusly. The blog statistics for today, for instance, showed that the earlier blogpost by Cynthia Lange on thusly was the second most frequently read post (it has been up for eighteen months already). Googling for the word myself showed that our blog comes up fourth in the results list, which would be part of the explanation.
But what makes people want to look up the word to begin with? Is thusly perhaps developing into a new usage item? If so, I’d be very interested to hear what makes people want to use it rather than thus if its status is indeed disputed.
Checking its frequency of use with the help of Google Ngram viewer shows that the use of thusly has greatly increased since the 1960s, reaching a peak shortly before 1980. The drop that followed wasn’t very great, and it looks as if it is there to stay. Is this considered a problem? What do text writers and editors think?
a search with Google Ngram viewer
Bridging the Unbridgeable 
If I saw “thusly” in something I was editing, I would strike off the “-ly”. I’m not sure it’s a very big problem, though; I’ve probably seen it only once or twice while editing.
Thanks, Jonathon. I found it in a (Dutch) student’s paper last night, and this added to my amazement about where it comes from. I corrected it as well.
I’ve looked up the usage problem ‘thusly’ in our Hyper Usage Guide of English (HUGE) database. As far as we have the data, since ‘thusly’ is on the list of usage problems we are currently entering into the database, ‘thusly’ occurs in usage guides from 1977 onwards. It is possible that we may find earlier entries as the work on the database progresses.
This fits in really well with the peak in usage that I found in the Ngram search!
There is also a downward trend of “thus”: https://books.google.com/ngrams/graph?content=thus%2Cthusly&year_start=1860&year_end=2008&corpus=0&smoothing=3&share=&direct_url=t1%3B%2Cthus%3B%2Cc0%3B.t1%3B%2Cthusly%3B%2Cc0
Likewise, I have started to see “seldomly” encroaching on “seldom”: http://katherinebarber.blogspot.ca/2012/01/things-are-seldom-what-they-seem.html
Interesting! The Google Ngram image for seldomly is just like that for thusly. I haven’t checked this yet (maybe Robin can do so?), but I wonder if seldomly, like thusly, is a usage problem, too.