Cripple Mr. Onion is a fictional card game mentioned in several of Terry Pratchett's Discworld novels. Although it gets mention in a number of them, it's only actually played for the reader aboard a riverboat in Witches Abroad. During a journey undertaken by the witches Magrat Garlick, Nanny Ogg, and Granny Weatherwax (forming the standard maiden, mother, and crone trio), Nanny loses their traveling money to a gang of hustlers playing Cripple Mr. Onion. Granny Weatherwax decides to hustle the hustlers and win their money back, in her own unique manner. It's during this sequence that we get our best, indeed our only, look at the game.
Resolving not to use magic (Cheatin's all right. That's practic'ly fair. I mean, anyone can cheat. But using magic — well, it's tempting Fate), Granny uses the principles she calls "headology" to distract, annoy, and agitate the card sharps to prevent them from cheating during the game, eventually winning their money back plus some extra. To Granny Weatherwax, we are told, losing was something that happens to other people. In the process, we find out the following few details about how to play.
The game in the book
Cripple Mr. Onion appears to be something of a cross between Poker and Hearts, so far as I can tell. Hands are not played quickly, one can spend "ages" building a good hand, and five minutes appears to be considered longer than average. How many cards are dealt is not revealed, but a reference is made to having nine cards at the most, and five at the least (more could possibly be in the hand if only some were of significant value, ignoring the others as irrelevant). An ante is paid before the hand starts, and there seem to be two ways to put more money into the pot: buying new cards and raising. It is not clear if cards are discarded when new ones are purchased or if the player just accumulates cards into a growing hand. A player can fold if he has a poor hand.
The only hand that is explicitly described is the Triple Onion (two kings and three aces), which is apparently a mid-range value. A three-card Onion (a different hand, apparently, from the Triple Onion) beats a double Bagel. A five-card Onion beats a Flush. A Broken Flush beats a two-card Onion (both of these are low-value hands). The best hand in the game is apparently a "natural Great Onion", which implies the existence of wild cards, and they're so rare that it's notable to get two in one day. A Great Onion can be beaten if another player can "cripple" it, but this is so rare that it would only happen once or twice in a player's life. This is done with a "perfect ninecard run", which appears to be akin to a straight flush.
The real-life game
Based loosely on these tidbits of information, Discworld fans have pieced together complete rules for a game of Cripple Mr. Onion that can be played in real life using an 8-suit deck (8 being a number of mystical significance on the Disc) of 104 cards. Rules are available in numerous places on-line and the game can be played with two to seven players. The game can be played with a deck referred to as a "Fat Pack": Spades are paired with Axes, Clubs are paired with Tridents, Hearts are paired with Roses, and Diamonds are paired with Doves. Oddly, the only suit mentioned in Witches Abroad, "Cups", is not in the Fat Pack, because this special deck of cards was not developed with Cripple Mr. Onion in mind, it's just one that can be used for it.
Hands are made of ten cards, dealt in two groups of five with a break in between to discard and draw new cards. The game is played by attempting to arrange some of your cards from your hand into the best combinations that sum less than 21 points. The lowest hand is a Bagel, the highest is a Great Onion. Gambling is optional. In the gambling game, purchasing a card means, during the second round of 5 dealt cards, you buy the right to have the card dealt face-down so the other players can't see it. There are 11 optional modifiers that can make the game more interesting, most of which are named after characters or concepts from the Discworld series.
Full rules available here: http://cripplemronion.info/