Until I find out the official name for such things, let us say a unigram is a word in which no letter is repeated.
Thus cat and dog are unigrams, but cattle and turtle are not, in both cases because the letter t is repeated. The definition ensures that a unigram can't be longer than 26 letters; it's easy to imagine that that are unigrams of 7, 8 or 9 letters, but the limit must not be far above that!
Can you come up with a unigram of 8 letters? What about 9 or 10? What is the longest unigram you can come up with?
I give up, show me the solution.