Ancient Greek farmers who grazed oxen and cows on their farms called both animals bous, the origin of "Bossy," our affectionate name for a cow. Bits of fat from Bossy's milk would rise to the surface, and they called this "boutyron." When the Romans invaded Britain in AD 43, they brought the Greek word with them. It eventually was modified to "butter" by the English. Salted butter became a staple on English sailing ships. By the 14th century, it was common in households.
For centuries, this American term has been used to describe a tall, slender person, like Abraham Lincoln. How so? Beans were first grown as animal feed (as were peanuts, another legume). Eventually beans appealed to people. But to grow the vines, gardeners found they needed tall poles. The gear was compared to tall, slender people.
SOURCES: 'What's in a Word' and 'Why You Say It,' by Webb Garrison; 'The Encyclopedia of Word and Phrase Origins' by Robert Hendrickson.