The question, of course, is in reference to the “Mother Goose” nursery rhyme about Miss Muffet, the earliest print date of which is 1805 in a volume called “Songs for the Nursery:”
Little Miss Muffet
Sat on a tuffet,
Eating her curds and whey;
Along came a spider,
Who sat down beside her,
And frightened Miss Muffet away.
“Perhaps some of you would like to know what a tuffet is. I have thought of that myself, and have taken the trouble to ask several learned persons. They assure me that the most complete and satisfactory definition is, — a tuffet is the kind of thing that Miss Muffet sat on.”
— Samuel McChord Crothers, 1902 
It’s not surprising if you don’t know what a tuffet is; after all, people have been arguing over this particular matter for at least 200 years. As a matter of fact, they have been arguing over an even more basic problem for that long… the question of whether or not tuffet was ever an actual word.
“Boy Blue: ‘What is a tuffet?’
Miss Muffet (jumping up from her stool and placing it before him): ‘That is a tuffet!’
Boy Blue: ‘That? Why, that’s nothing but a little footstool! What makes them call it a tuffet?’
Miss Muffet: ‘Because tuffet rhymes with Muffet, stupid, and footstool doesn’t!'”
— S.J.D., 1898 
Get the point? Just over 100 years ago, many people were already of the opinion that the ‘word’ tuffet was simply a nonsense made to rhyme with Muffet; therefore, what it actually meant was beside the point! Little wonder then that now a tuffet is, if regarded as a real word, assigned a variety of meanings; each of which has been asserted as the right one. This confusion and speculation is all due to a simple lack of information… for some reason, the word tuffet doesn’t seem to have been included in dictionaries previous to about 1780, and by then the word was no longer in regular use… so even the dictionary definitions at this point were viewed with suspicion.
So, of all the modern ideas regarding the identity of a tuffet — a clump of grass, a small hillock, a stool, and/or a pillow — which, if any, are right? I have found one reference — just one — to the word tuffet predating 1780. The following is from a book about plants published in 1578, and is part of the description of a plant called the Royall Satyrion (which we now call an ‘orchid’):
“…ye floures grow in a spiky bushe or tuffet, at the top of the stalke of a light purple colour, and sweet savour…” 
So, by implication, we learn two things: first, a tuffet is a kind of a bush (so supporters of the clump of grass theory win), and secondly, it’s possible that the rhyme of Little Miss Muffet actually dates back as far as the 1500’s, when the word tuffet was in use!
1) “Miss Muffet’s Christmas Party,” by Samuel McChord Crothers, 1902 The Riverside Press Cambridge, Boston and New York, New York, USA. Pg. 4.
2) “Christmas Eve at Mother Hubbard’s,” by S.J.D., a story in St. Nicholas Magazine, Vol. XXV, No. 3, January, 1898 The Century Co., New York City, New York, USA. Pg. 254.
3) “A Nievve Herball, or Historie of Plantes,” by Rembert Dodoens & Henry Lyte, 1578. Pg. 225.