The Northern-English noun butty, which now denotes a filled or open sandwich, originally denoted a slice of bread spread with butter.

This noun is composed of:
– butt-, from the noun butter;
– the suffix -y, forming diminutive nouns.

Curiously, both the texts containing the earliest occurrences of the noun butty that I have found are associated with poisoning—accidental poisoning in one case, intentional poisoning in the other.

The earliest occurrence of the noun butty that I have found is from The Lancaster Gazette; And General Advertiser for Lancashire, Westmorland, &c. (Lancaster, Lancashire, England) of Saturday 14^th April 1827:

Caution to Parents.—Two Children Poisoned!—On Thursday week inquisitions were taken at Liverpool, touching the deaths of Ellen and Mary Johnson, the former aged four years, and the latter eighteen months.—The parents of those infants—Lawrence Johnson, a labourer, and Mary, his wife—live in a cellar in Gascoyne-street; and it appeared that their daughter, Elizabeth Johnson, a girl about ten years of age, playing in a field near Chisenhale-street Bridge, on Tuesday, saw a boy gathering some roots, which grew by the side of a pit, and thinking they were celery roots, she begged him to give her one. He did so, and she took it home and gave it to her aunt (Ellen Williams, sister of the children’s mother) who placed it in the window, where it remained until between five and six o’clock on Wednesday afternoon. About that time, the aunt was sitting by the fire, with the younger infant, Mary, on her knee, and Ellen asked her for “a butty,” meaning some bread and butter. There being neither bread nor meat [note 1] of any kind in the cellar, she evaded the application, and shortly afterwards Ellen took the root before-mentioned out of the window, and biting a piece of it, gave it to Mary, who ate it. The aunt herself also ate a small piece of it; and the two infants ate the remainder of the root between them, Ellen eating the greater part. The aunt states, that she had never eaten raw celery before, and did not know the difference in its taste from what she then ate, nor did she find any thing unpleasant in the taste of what she ate. Shortly afterwards, Ellen having gone out and seated herself upon the steps of the next door, with Mary in her lap, was seen to fall upon the pavement by a neighbour, who, supposing it to have proceeded from carelessness, blamed her, and told her aunt. In a minute or two afterwards another neighbour saw her fall again, and took both the children into the cellar to the aunt. They then fell into fits, and application being made to the North Dispensary for assistance, Mr. James Dixon, the House-surgeon’s Assistant, attended, and finding their bodies much distended, said he was sure they must have eaten something which disagreed with them, and begged to be told what they had taken. The aunt then handed from the fire-place a green leaf or two of a plant, the root of which she said the children had eaten. He at first thought it was celery, but upon examination found it to be hemlock [oenanthe crocato [note 2], or water drop-wort,] which is a narcotic poison, and the quantity mentioned by Ellen Williams was more than sufficient to have caused the death of the children. Ellen died immediately after the arrival of Mr. Dixon, and he endeavoured to administer an emetic to Mary, but she could not swallow, and immediately afterwards she fell into a state of insensibility, and died in about twenty minutes. The aunt took a strong emetic, and sustained no further injury. On opening the bodies of the infants, the symptoms observed were such as to leave no doubt that the hemlock which they had eaten was the cause of their deaths; and the jury returned a verdict to that effect.

The second-earliest occurrences of the noun butty that I have found are from The Chester Chronicle and Cheshire and North Wales Advertiser (Chester, Cheshire, England) of Friday 6^th August 1841:

THE STOCKPORT POISONING CASES.

[…]
Robert Sandys, 25, Ann Sandys alias Devanah, 25, George Sandys, 28, and Honor Sandys, 28, had been committed at a coroner’s inquisition and true bills found for the murder of three children, Elizabeth Sandys, Catherine Sandys, and Mary Ann Sandys, by poisoning, at Stockport. […]
[…]
The Attorney General stated the case to the effect, that the indictment on which he proceeded was for the murder of Mary Ann Sandys, by administering arsenic in bread and butter, on the 12^th of October last, so as to produce death on the 13^th; and the prisoners R. Sandys & Ann Sandys alias Devanah, charged as principals, and the other two as accessories, both before and after the fact. The prisoners were all Irish, and followed the occupation of bear makers [note 3]. The two first prisoners had four children, which were all entered in the Philanthropic Benefit Society at Stockport, and a penny per head per week was paid. When seventeen pence on each child had been paid, then it became entitled to the benefit of the club, and if it died the parents received £3. 12s. as burial money. On the morning of the 11^th of October the child Mary Ann, the subject of this enquiry, was perfectly well, and had some tea given it by a woman named Bridget Riley, its godmother. At ten o’clock it was still well, and was seen playing with two of its brothers and sisters. At eleven o’clock the children got some sand for a neighbour, and were then well. About twenty minutes before twelve, the child Mary Ann became ill. One of the other children—Robert Sandys, who had been instructed as to the nature of an oath, since the last assizes—would say, that Ann Devanah, the mother, gave Mary Ann a butty.
[…]
Robert Sandys, the son of the prisoners Robert and Ann Sandys, was next produced. He appeared not more than six years of age. The learned Judge put several questions to him to test his knowledge as to the nature of an oath, the penalty of telling lies, human responsibility, &c. &c., which were answered to his lordship’s satisfaction, and he directed him to be sworn. He said:—l remember my sisters, Mary Ann, Jane, and Elizabeth; Elizabeth is not living; I remember Mary Ann being taken ill, and dying; I went for sand that morning, and Mary Ann, Jane, and Edward, were with me; I know Bridget Riley; I saw Mary Ann and Jane go towards Bridget Riley’s; I was in Handford’s yard with my sisters; we went for sand to Sarah Lees; Mary Ann and Jane went with me; my sisters and I went into the cellar; Edward was there; saw my father there; my mother was there also; we had had meal porridge and milk for breakfast; we all had some bread and butter after that; my mother gave it to us; it was after going for the sand; I eat my bread and butter; it did not make me sick; I did not see my sisters have any other bread and butter given them; their’s [sic] was taken from the same loaf as mine; salt was put to Mary Ann and Jane’s butties; none was put to mine; they eat their butties; my father was in the cellar; my mother cut the butties ; we went out after; we did not go into any house.

Notes:
1 In “neither bread nor meat of any kind”, the noun meat has its original sense of food in general, anything used as nourishment, solid food as opposed to drink. This original sense survives in sweetmeat and in the phrase be meat and drink to, meaning be a source of great pleasure to.
2 Misprint for Œnanthe crocata.
3 The noun bear is a Northern-English name for a doormat.