Irregularized jack-knives, regularized jack-knifes

Caught in passing in some tv commercial: “It never jack-knives”. With jack-knives as the 3sg PRS of the verb jack-knife — though the standard verb form is jack-knifes. The irregular verb form is obviously modeled on the irregular PL jack-knives of the *noun* jack-knife, inherited from its head noun knife (PL knives); the 3sg PRS of the verb knife is the regular knifes.

A few examples of the verb jack-knives from the net:

I was just sort of waiting until the day he fell on to the couch and the frame gave way, like in movie when someone jumps into a bed and it jack-knives on them. (link)

The 18-wheeler makes as radical a turn as a vehicle that size can. It JACK-KNIVES into a 90-degree angle upon itself. (link)

The fix is to replace the hex metal block on the gearbox end of the selector cable with a rose joint (or similar) to allow some lateral movement (otherwise it jack-knives a bit at the end of its travel and can be sticky). (link)

On the verb jack-knife, from OED2:

jack-knife v.  (a) trans. to cut with a jack-knife;  (b) intr. to double up like a jack-knife; spec. of the sections of an articulated lorry: in an accident, to fold together like a jack-knife;  (c) to do a jack-knife dive.

[first cite] 1806   Balance (Hudson, N.Y.) 22 July 228   A sailor..Jacknifed (as he termed it) the poor creature [sc. a cat] in several places about the head.

[cite with 3sg PRS] 1920   T. S. Eliot Ara Vos Prec 22   The sickle motion from the thighs Jackknifes upward at the knees Then straightens down from heel to hip.

The irregularized verb is attested, but not enormously common. What’s more common is the regularized noun. First, there are cases where the noun jack-knife doesn’t refer to a knife; regularization in such cases is very common. A few examples, from telephony, wrestling, and exercise (there are probably also examples from diving):

[telephony] jack-knife (Electricity)  A form of commutator used in telephone central stations. … To connect the two jack-knifes of the subscribers, right and left, a flexible metallic cord terminating in two pins is used. [1884 dictionary of tools, instruments, machines, etc.]

[wrestling] Kevin Nash hit two Jack-knifes and Booker T hit Two Book Ends and Nash rebounded Booker T against the ropes and hit the big boot and signaled the Jack-knife while the ref wasnt looking (link)

[exercise] Okay, the biceps section is a bit cruel. Two counts up and down for an eternity of reps. And I could only do one of two jack-knifes on the ball – but I expect to improve! (link)

Regularization isn’t inevitable in such cases; here are irregular plurals corresponding to the last two regular ones:

[wrestling] Diesel delivered two Jack Knives and had the Undertaker flat on his back. (link)

[exercise] If you can’t touch your legs without jerking at first, go as far as you can go. It will come in time. At first you may find you can only do one or two jack knives. (link)

In one case (jack-knife as a truncation of jack-knife sofa), regularization strikes me as nearly obligatory:

[one poster] I would like to replace my jack knife sofa the one that came in the TT [travel trailer] is pretty uncomfortable to sit on and sleep on.

[another] We have two Jack Knifes that fold into each other. They are really comfortable. (link)

In addition to these motivated examples of regularization, there are some instances of regularization of jack-knife referring to a knife:

[story published by The Daily Ypsilanti Press of Tuesday, May 3, 1927] Beside the body were found a man’s belt buclke with the initial E., some over all buckles scissors, looking glass, two jack knifes, a dime and a penny. (link)

[recent fiction] They both scrambled through the stock boxes and managed to recover a foldable tent, two mountain climbing backpacks, three long range rifles, two jack knifes, and a hunting bow with 8 arrows. (link)

So we have both irregularization of the verb and regularization of the noun.


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