Dude.
Bruh!
Broski…
Okay, I’ll make a final attempt…
In the joke, the ‘thinkable’ is actually ‘sinkable’.
Which is not clear when you say it the first time because it sounds like you’re saying ‘think’ & ‘thinkable’, both of which are actually words.
Trick lies in enunciating the punchline.
Hence the Mike Tyson reference.
Now, if you still haven’t got it, I really hope you are very very very rich so that you can survive in this world with that super smooth brain in your skull.
If you did get it now, henceforth it is your ethical and moral duty to spread this stupid-ass joke every time you get the chance.
God speed and be weird.