Thusly

When did "thusly" become a word?

My trusty old Merriam Webster edition ten with a 1995 copyright says that thus and thusly mean the same thing, and both can be defined thusly: "in this manner."

If one were to ask, "In what manner?", we'd be stuck in a loop--like when "Pete and Repeat sat in a boat--Pete fell out; who was left?"

Thus and thusly are also both adverbs, which throws me a little more, because I find it difficult to define adverbs. They modify or qualify...um...well, just about anything--adjectives, nouns, other adverbs, other collections of words, snow, birds, space, time, and so on.

Word use charts on the internet suggest a gradual increase from around 1900 for thusly, whereas thus has much more use back to 1800 and before.