What English word can be repeated an arbitrary number of times to form a grammatically and semantically meaningful English sentence?

Buffalo has two meanings which are sufficient to solve the problem (though Webster 1913 only lists one: first, the common noun, meaning a large mammal, and second, a transitive verb, meaning to intimidate or to coerce. For the sake of clarity, in the following example sentences I will substitue the word bison for the animal and use buffalo exclusively as a verb.

This is a proof by induction. I will give examples of setences of length 1-3 and then give a description of how to take a sentence of length >= 3 (where length is an odd number) and form sentences of lengths length + 1 and length + 2

Length 1: "Buffalo!" is an imperative exclamation using the infinitive form of the verb.

Length 2: "Bison buffalo." is an existential statement that asserts that large four-legged mammals intimidate or coerce. (note that I will not promise that all of these sentences are, in fact, true, only that they are intelligible.

Length 3: "Bison buffalo bison." is an extension of our length 2 sentence that quantifies who the recipient of the coersion or intimidation is.

Length 4: "Bison bison buffalo buffalo." is the trickiest sentence to interpret. "Bison bison buffalo" is a noun phrase meaning, roughly, "The animals who are intimidated by other animals", and our sentence asserts that those intimidated animals are also guilty of intimidating (though just who they intimidate is not clear).

Length 5: "Bison bison buffalo buffalo bison." is the natural extension of our length 4 sentence, adding a direct object to the verb we added at the end of the last sentence.

So now we can create longer sentences by moving the direct object to the beginning of the sentence, creating a noun phrase ("The bison whom bison who bison buffalo buffalo...") and adding a verb to the end to make a sentence of length + 1, or adding a verb and a direct object to make a sentence of length + 2.

Please don't blame me if you get some strange stares from people when you tell them that buffalo buffalo buffalo buffalo buffalo.

_{Note: see also this node. There is some redundancy between that node and this one.}

It was conceived by William J. Rapaport, professor at the University of Buffalo, while an undergrad at Indiana University, and featured in Steven Pinker's The Language Instinct.

Buffalo buffalo (bison in Buffalo) Buffalo buffalo buffalo (who other Buffalo bison bully) buffalo (will bully) Buffalo buffalo (bison in Buffalo).

However, to make this clear, punctuation should be included:

Buffalo buffalo, Buffalo buffalo buffalo, buffalo Buffalo buffalo.

Um... I think.

Rappaport explains it this way:

Then, of course, you can make it more interesting by considering the buffalo in the Buffalo zoo, the Buffalo buffalo. And their peculiar way of buffaloing other Buffalo buffalo, so peculiar that, like the Tennessee waltz which you do by Tennessee waltzing, they Buffalo buffalo those other Buffalo buffalo:

Buffalo buffalo Buffalo buffalo Buffalo buffalo Buffalo buffalo Buffalo buffalo.

TO see the original:
http://www.linguistlist.org/issues/3/3-175.html#1

WaldemarExkul noted to me:

Moreover, *any* number of consecutive instances of the word buffalo is a grammatical English sentence. ("Buffalo!" - 'Go out and bully (people/things)!' - "Buffalo buffalo!" - 'Bully bison!'; "Buffalo Buffalo buffalo!" - 'Bully bison from the city of Buffalo!'; "Buffalo buffalo buffalo buffalo." - 'Bison from Buffalo bully bison.'; and so on and on...)