The joke, which was new to me and entertained me enormously:
Then, passed on my a friend yesterday, from Facebook, the joke supplied with a photo, the result then labeled as a “meme” (in my terms, since the text is the crucial element, this is a texty creation, or just a texty, a cousin of cartoons):
The joke turns on an ambiguity in the communicative intent of This is a library, which is merely an assertion that the place the two conversants are in is a library. The question is why the librarian asserts this to the patron. Surely the patron knows that already.
However, we are to understand the librarian’s response as reproachful. But for what reason?
Here, we need to supply some sociocultural background, about the nature of libraries, as places where, in addition to borrowing books and other materials, patrons sit and read to themselves, a practice that normally calls for quiet, so it’s common for librarians to remind patrons of that requirement — stereotypically, by shushing patrons who speak too loudly. That is, we would normally suppose that a librarian would be reproaching a patron for the form of their speech: her response would convey ‘that’s not the way we use our voices in a library’.
However, the content of the patron’s speech in #1 and #2 is quite remarkable in the context of a library: he appears to expect to find access to hookers and booze there. Now, in the real world some libraries have on occasion been afflicted by both drinking and sexual activity in the stacks, but no library provides these as actual services. Given that, the librarian is clearly reproaching the patron for his preposterous expectation: her response conveys ‘that’s not what a library is for’.
The patron, however, disregards the clear communicative intent of the librarian and chooses to respond as if it were a shush, so repeats, verbatim, his preposterous announcement that he’s looking for hookers and booze — but in a whisper. Absurdly, fixing the form but not the content.
Apparently, he’s so much in need of booze and hookers that he’s lost his grip on how libraries work. That makes him a stock figure of jokes, and comedy in general: the narcissistic fool — insensitive to his surroundings, responsive only to his own aims and desires. A figure of fun. (Of course, in the real world, such people are moral monsters, and they’re dangerous.)