You might think this word simply means “to go with quick steps on alternate feet, never having both or (in the case of many animals) all feet on the ground at the same time.” But no such luck: that is merely sense I.1a, and there are miles to go before the reader of this particular entry may sleep.

It took Peter Gilliver, the O.E.D. lexicographer working on the letter R, more than nine months harnessed to the duties of what Samuel Johnson once called “a harmless drudge” (plus many more months of preparatory research) to work out what he believes are all the meanings of “run.” And though some of the senses and their derivations try him — Why does a dressmaker run up a frock? Why run through a varlet with a sword? How come you run a fence around a field? Why, indeed, run this essay? — Mr. Gilliver has finally calculated that there are for the verb-form alone of “run” no fewer than 645 meanings. A record.

In terms of sheer size, the entry for “run” is half as big again as that for “put,” a word on which Mr. Gilliver also worked some years ago. But more significantly still, “run” is also far bigger than the old chestnut “set,” a word that, says Mr. Gilliver, simply “hasn’t undergone as much development in the 20th and 21st centuries as has ‘run.’ ”

But why? The decline of “set” is the more readily explicable. Basically “I think ‘put’ killed ‘set,’ ” another lexicographer mused to Mr. Gilliver in an only-in-Oxford pub chat recently, noting that you now put a vase on the mantel, rather than set it there, and put words on paper, not set them.

Explaining why “run” has so greatly expanded its semantic territory is more difficult. The word has exploded with the increase in the number of machines and computers: a train runs on tracks, a car runs on gas, an iPad runs apps. But simultaneously, there have also been countless revivals of antique non-mechanical senses: you now run out on someone, you run something past someone. Old “runs” are, in other words, generating new meanings, a demonstration of the living nature of the language.