it has no butter or other dairy ingredients in it so why call it butter
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Within the world of sugar preserves, there is a difference between jams, marmalades, and butters. Has to do with the consistency of the ingredients, and the proportion of sugar to other ingredients.
But that still begs the question as to why "butter" in the first place?
But from what I can find, the name "peanut butter" is mostly a marketing gimmick. From The Oxford Encyclopedia of Food and Drink in America:
For centuries peanuts have been ground and consumed by indigenous peoples of South America and by Africans, but peanut butter was not popularized in America until the vegetarian John Harvey Kellogg endorsed it as a substitute for "cow's butter." In the early 1890s Kellogg crushed various nuts between two rollers and claimed the results to be "nut butters." . . . Kellogg was an excellent promoter. He extolled the virtues of peanut butter throughout the nation. To commercialize his discovery, Kellogg created the Sanitas Nut Food Company and placed his brother, Will Kellogg, in charge. Nut butters quickly became a fad among other health-food manufacturers in America.
-- Andrew F. Smith
In theory, any fleshy fruit can be turned into a butter. According to the Ball Blue Book, "Butters are made by cooking fruit pulp and sugar to a thick consistency that will spread easily. Spices may be added; the amount and variety depend upon personal taste. After sugar is added, butters should be cooked slowly and stirred frequently to prevent scorching. If a fine-textured butter is desired, the pulp can be processed through a food mill and then strained through a fine-meshed sieve."
That Andrew Smith quote certainly sounds like a reasonable explanation. And fits the merchandising mode of many 19th and early 20th century health-food proponents---both serious ones and charletens both.
A little harder to spread actually.
There are many fruit butters, pear being my favorite.
They spread smoothly and evenly, like softened butter.
For the inevitable question regarding chunky peanut butter, that came after the product was named.
Just as there was no such thing as a black & white TV until color television was invented, there was no such thing as smooth or creamy peanut butter until chunky hit the market.
Evidence of modern peanut butter comes from US patent #306727 issued to Marcellus Gilmore Edson of Montreal, Quebec, Canada in 1884, for a process of milling roasted peanuts between heated surfaces until the peanuts reached "a fluid or semi-fluid state." As the product cooled, it set into what Edson described as "a consistency like that of butter, lard, or ointment."