I find great joy in people being confused by Alastor picking up new slang.
Like this was his job, that he greatly enjoys by the way. His job is to observe people, look for things he could scrutinize, maybe look for things his audience would specifically like to listen about. He was literally paid to observe people.
I feel like all the time spent in speakeasies and high class parties was not only for him to look for a next target that was some of it but for him to look for things to yap about as well. And don't get me STARTED on the amount of tea this man has heard through the years.
I've also been thinking about this meme. I know it's a meme and not serious but I still wanted to yap about it. This is also apart of his job. If the old timeline is correct and he was in his 40's when he died. Even if he started his career late into his 30's, that's still about a decade of new slang he had lived through as a human. Think about how much of our own slang changes just from year to year. Learning new words is child-play to this man.
Alastor picking up new slang through the decades in hell would not be surprising, if anything it would be expected. He could even pull off like decade dedicated broadcasts throughout the week. "Ha ha! Hello all of my 'Groovy cats' out there today! I've got all the 'happenin's' for those stuck in the 60's! Now enjoy this next track from the 'fab four'!"
Alright auntie is entitled to her nap now. Chew this one over friends! <3