This is a strange expression, often heard in the form: "You'll be laughing on the other side of your face [when X happens]." But what does it mean and where does it come from?

Its meaning is straightforward enough. To laugh on (or out of) the other side of your face (or the wrong side of your face, or mouth) is to experience a humbling reversal of fortune – to have your happiness or amusement change to sadness, annoyance, hurt, disappointment, etc. There is often an implication that the change is deserved.

The expression can be used in a threatening or vengeful way, e.g., "I'll make you laugh on the other side of your face!", which is similar to "I'll wipe that smile off your face!" Jenny at Yahoo! Answers reports: "In Dublin, where I grew up, it meant you wre [sic] going to get a good whack on the side of your gob!! lol." The image of a violent action knocking a cheeky smirk away – right across the face or entirely off it, Picasso-style – is exploited literally in cartoons, and is acknowledged in Graeme Donald's Dictionary of Modern Phrase:

It might be that, since mocking laughter usually issues from one side of a twisted mouth, the expression implies a threat to twist the mouth to the other side by administering a blow.

But the laughter isn't necessarily mocking, and the comeuppance is often non-violent. The phrase is commonly used in a situation where person A is simply pleased or confident about something, and person B finds A's complacency inappropriate or inopportune. B believes that things will change and that A will not be amused for long. So B says something like, "You'll soon be laughing on the other side of your face."

The Cambridge Advanced Learner’s Dictionary says the phrase is "used to tell someone that although they are pleased now, they will not be pleased later when things do not happen as they expected or planned"; the OED offers a concise equivalent, that an other-side-of-the-face laugher is "discomfited after premature exultation." Brief definitions at Brewer's, Oxford, Merriam-Webster and Macmillan support these senses, as does the following entry at Random House:

to undergo a chastening reversal, as of glee or satisfaction that is premature; be ultimately chagrined, punished, etc.; cry: She’s proud of her promotion, but she’ll laugh out of the other side of her mouth when the work piles up.

It's not uncommon in literary contexts, especially in dialogue. You can see it used to various but related ends in Death of a Salesman, The King of Ragtime, The Epic of Latin America, Freaks and Follies of Fabledom, War Plays by Women, and The Fall of Fort Sumter, or, Love and War in 1860–61 (“I’ll make the whole country laugh on the other side of their mouth”). Searching for variations in Google Books will turn up many more examples.

The second question – Where does it come from? – is unresolved, as far as I can tell. It's quite an old idiom, dating back to the late 1700s at least. But its very familiarity obscures the bizarre conceit of this anatomical metaphor. Laughing on the other side of your face: what are we to make of this? Is it a reference to Janus? To the masks of comedy and tragedy? On the Yahoo! Answers page, Grannyjill, noting that people don’t just use the left or right side of their face when they laugh, wonders: “So what other side? The side next to the bones?”

My query on Twitter led to a few replies and ideas, but nothing definitive. Embedded in the phrase is an inversion of the fates, a foretelling of the fall that follows pride. Maybe the cartoons have it right, and it's more directly literal, and rooted in violence. Anecdotally I've seen it associated with parental discipline several times. Pet theories, however (im)plausible or outlandish, would be welcome.