Shaftment

The width of a closed fist with the thumb extended, reckoned at six inches.

Old English sceaftmund, and one of the oldest body measures in the language. Close the hand, extend the thumb, and the distance across is a shaftment: about six inches on most adults, and two of them to the foot.

Its value was that it could be used on a thing already held. A man gripping a shaft, a spear or an oar could step the length off against his own fist without putting the object down or finding a rule, which is a different kind of usefulness from the span or the cubit and probably explains the name.

Like the hand, the span and the thumb it is a measure that varies with the man using it, and that was tolerable for as long as the work was done by the man who had measured it. It fell out of use as soon as anything needed to be made by one person to a length specified by another.