The meaning behind the proverb is also subject to debate among scholars. Gordon suggested that the inn also apparently served as a brothel (he notes that the word used in the proverb for inn or tavern, “éš-dam”, can also be translated as “brothel”, and it was common in ancient Mesopotamia for prostitution to take place in these establishments[3]), and thus “the dog wanted to see what was ‘going on behind closed doors’”.[4] Nett suggests that the punchline could be a pun that is incomprehensible to modern readers, or a reference to some figure who was well known at the time but similarly unfamiliar to modern readers. Gonzalo Rubio, another Assyriologist, cautions that this ambiguity ultimately means it is simply not possible to definitely categorize the proverb as a joke, though he and other scholars like Nett do point to the recurring use of innuendo in such proverbs as indicating that many were indeed intended to be humorous.[3]

Just pasting for people as lazy as me.

Tldr: nobody knows what the joke is, which itself is the joke.