I remember reading A Clockwork Orange for the first time and I felt something similar. The gang in the story uses heavy street slang, which is uninelligible at the start. After a few chapters of immersion in context, you become fluent and I found bits starting to slip into my own vernacular.

It's a testament to our amazing brain's ability to adapt. But, it is also cautionary, as we are always absorbing, even unconsciously.