martingale

Martingale breeches: breeches held to belt with buttons and points, having a movable panel between legs. (Renaissance)

Makes me think of farthingale.

In fencing: a strap attached to the sword handle to prevent a sword from being dropped if disarmed.

"Martingale, in a ship, a name given to the rope extending downwards from the jib-boom end to a kind of bumkin, and generally fixed perpendicularly under the cap of the bowsprit; its use is, to confine the jib-boom down in the same manner as the bobstays retain the bowsprit."
—Falconer's New Universal Dictionary of the Marine (1816), 262

A betting game with a long history, seen here.

A martingale also is a piece of tack used in training horses. It helps them learn to keep their head down when jumping.

"In all the local cafés he gave out his visiting card, which described him as "Head of Practical Services at the Ecole Pyrotechnique," and he offered his services generously; he obtained innumerable orders for superactive hair and carpet shampoos, stain-removers, energy-saving devices, cigarette filters, martingales for 421, cough potions, and other miracle products."
-- Life: A User's Manual by Georges Perec, translated by David Bellos, p 23

Usages:
"Captain Aubrey pondered, staring at the dolphin-striker." (379)
"'How do you find your martingales answer, led single like that?'" (380)

"The captains were back at their martingales and dolphin-strikers when a tiny shrill young gentleman... came running forward and said 'Uncle William, she wants you in the cabin.' Then recollecting himself and blushing he pulled off his hat and said 'If you please, sir, the lady in the cabin's compliments to Captain Babbington and would be glad of a word with him at his leisure.'"
—Patrick O'Brian, The Surgeon's Mate 380

A stochastic process in which the conditional expectation of the next value, given the current and preceding values, is the current value.

Originally, martingale referred to a class of betting strategies popular in 18th century France. The simplest of these strategies was designed for a game in which the gambler wins his stake if a coin comes up heads and loses it if the coin comes up tails. The strategy had the gambler double his bet after every loss, so that the first win would recover all previous losses plus win a profit equal to the original stake. Since a gambler with infinite wealth will with probability 1 eventually flip heads, the Martingale betting strategy was seen as a sure thing by those who practised it. Unfortunately, none of these practitioners in fact possessed infinite wealth, and the exponential growth of the bets would eventually bankrupt those foolish enough to use the Martingale. Moreover, it has become impossible to implement in modern casinos, due to the betting limit at the tables. (This strategy is related to the St Petersburg paradox)