A lot of people think that slang is a modern invention, but most people also know that words like Good-Bye derive from God Be With Ye, or that Howdy originates from “How do you do?”, which is how we end up with Howdy do or Howdy doody. But even in the medieval period, there was slang. In Thomas Mallory’s Le Morte d’Arthur, I found this passage.
“All the three kings and the whole barons said that merlin said passingly well, and it was done anon as Merlin had devised” - Book 1, Chapter XIV
It’s easy looking back at middle English and being confused by it, after all, it is a different language from modern English. But in that passage, the word Passingly isn’t just an old fashioned outdated example of grammar or lingual drift, it’s a slang way of saying “Surpassingly”. And passing and passingly as slang for surpassing and surpassingly date back to the 14th century.