jackknife

n.

1. A large clasp knife.
2. Sports. A dive in the pike position, in which the diver straightens out to enter the water hands first.

v., -knifed, -knif·ing, -knifes.

v.tr.

1. To fold or double (something or oneself) like a jackknife.
2. To cut or stab with a jackknife.

v.intr.

1. To bend or fold up like a jackknife: A truck that had jackknifed was blocking the road.
2. To form a 90° angle.

A computer-intensive resampling method for estimating some unknown parameter of a distribution while making minimal assumptions. In this respect it resembles the bootstrap. Denote the parameter by θ and its usual estimate, based on a sample of n observations, by θ̂. For example, if the parameter were the mean of a distribution then the usual estimate would be the sample mean.

The jackknife procedure produces an alternative estimate θ̃, together with an estimate of the bias (if any) of the usual estimate. Let θ̂_−j be the usual estimate of θ calculated from the same sample but with the jth observation omitted. Now define the pseudovalue θ̃_j by θ̃_j = nθ^ - (n - 1)θ^_-jThe jackknife mean θ̃ and variance s² are given by

. The estimated bias is θ̂-θ̃. The ratio


has an approximate standard normal distribution. The method also applies to the estimation of more complex characteristics, such as the correlation in a set of bivariate observations. The term jackknife was coined by Tukey in the early 1960s.

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Origin: 1711

Americans may not have been the first to think of making a knife safe and portable by giving it a blade that folds into the handle, but we were the first to call it a jackknife. Our earliest notice is in the official records of Springfield, Massachusetts, for 1711: "One Dozen of Jack Knives, at six pence the knife" on one occasion, and "Eleven iron handled Jack knives" on another.

Jackknives were a traditional accouterment for boys who were growing up in America. Without them the playing of mumble-te-peg (an English game which antedates jackknife by eighty years at least) and the whittling of sticks (which probably antedates English and perhaps even the Bronze Age) can only be practiced by the possessors of the larger and more formidable sheath knife (1837). The practical advantage to the jackknife lies in its relatively safe transportability in one's pocket.

Where did we get the term? Perhaps it came from the Scottish word for a similar knife, jockteleg knife or Jock the Leg Knife, attested as early as 1672. Or perhaps it was from Jack meaning "sailor," since the knife was used by sailors.

In modern times, the figure of a jackknife opening and closing has found application beyond the tool itself. Its name has been given not only to a style of diving (1922), but to a highway accident where a tractor-trailer truck folds in on itself.

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IN BRIEF: A large pocketknife. Also: To for a 90-degree angle.

The truck driver jackknifed his rig on the highway when he slid on ice.

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Dansk (Danish)
n. - lommekniv, foldekniv
v. tr. - skære med en lommekniv
v. intr. - folde sammen som en lommekniv

Français (French)
n. - couteau de poche
v. tr. - se mettre en travers
v. intr. - se mettre en travers

Deutsch (German)
n. - Klappmesser, Taschenmesser, Hechtsprung
v. - sich querstellen, einen Hechtsprung ausführen, zusammenklappen, sich zusammenfalten

Ελληνική (Greek)
n. - μεγάλος σουγιάς
v. - (καθομ.) διπλώνω/-ομαι στα δύο, (Βρετ., καθομ.) (για νταλίκα) κάνω ψαλίδι

Italiano (Italian)
temperino, coltello a serramanico, sbandare

Português (Portuguese)
n. - canivete (m)
v. - dobrar-se

Русский (Russian)
складной нож, резать, согнуться

Español (Spanish)
n. - navaja, salto de la carpa
v. tr. - hacer el salto de la carpa, colear (un camión articulado), cortar con navaja
v. intr. - doblarse en el medio como en el salto de carpa, agacharse

Svenska (Swedish)
n. - fällkniv, pikhopp
v. - utföra ett pikhopp, slira åt sidan (om släp)

中文（简体）(Chinese (Simplified))
大型折合式小刀, 镰刀式跳水, 把...折拢, 折刀似地折拢, 作镰刀式跳水

中文（繁體）(Chinese (Traditional))
n. - 大型摺合式小刀, 鐮刀式跳水
v. tr. - 把...摺攏
v. intr. - 摺刀似地摺攏, 作鐮刀式跳水

한국어 (Korean)
n. - 큰 접지칼, 해군용 칼
v. tr. - 큰 접지칼로 지르다, ~을 접다
v. intr. - 급히 꺾다, 급히 구부러지다

日本語 (Japanese)
n. - ジャックナイフ
v. - ジャックナイフで切る

עברית (Hebrew)
n. - ‮אולר גדול (מתקפל), קפיצת אולר - קפיצה ממקפצה בגוף כפוף והתיישרות‬
v. tr. - ‮התקפל כמעט לשניים (כאולר)‬
v. intr. - ‮התקפל כמעט לשניים (כאולר), קפץ קפיצת אולר‬

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