Anyone who has been online a lot knows of the Sumerian bar joke, because it's gone viral in memes more than once. Found in a collection of Sumerian proverbs and aphorism, it is proverb 5.77:
A dog entered a tavern and said: "I can't see a thing. I'll open this one!"
Because of the structure it is often called the 'first bar joke'. There are many speculations about what this might mean. A very popular one is that the dog's eyes are closed. Another is that there is a series of sexual innuendoes. The word for 'tavern' is sometimes associated with prostitution (not necessarily a brothel, but someplace you might find prostitutes), for instance.
But I wonder. While it likely has a humorous edge (many of the Sumerian proverbs we have seem to), and could have sexual innuendo (a number of them do), most of the aphorisms in the collections have a typical didactic point you would expect proverbs from a proverb collection to have. So we would expect this one to capture a lesson of some sort, as well.
It's natural when reading the proverb to take it to give a sequence -- the dog enters the tavern and then says something. But it may make more sense to see the dog's comment as explaining its entry into the tavern. There is another, recurring proverb we find in the collections (3.26, 19.c2, 24.9):
When the sun is setting outside so that you cannot even recognise the hand in front of you, go indoors!
The humorous point is perhaps then that the dog (often a symbol in the aphorisms for a self-indulgent person) picks the tavern as the 'indoors' when going inside for the night. There are two possibilities:
The dog is foolish. Doing something apparently common-sensical (it's too dark to see, so it goes inside), the dog has no sense and therefore opens the seediest possible door in order to get inside.
The dog is disingenuous. The dog is doing something apparently reasonable, but it has rigged it up so that it will be outside a tavern when it's time to do it. He's covering, by an excuse, the fact that he is actually just indulging himself. In this sense the idea might be that a vicious person uses advice as an excuse to indulge his vices. Alternatively, the dog might just be nosy. There is another Sumerian proverb in the collections (2.109),
A sniffing dog entering all houses,
which seems to convey the idea of a nosy person. Thus the dog is trying to pretend it's just being reasonable in indulging its nosiness.
In any case, it's anyone's guess. In interpreting such things, we're a bit like Athanasius Kircher trying to decipher hieroglyphics.