Early Italian Renaissance painters were extremely popular in their time, but not much is known about how they worked. There is no evidence that they used any sort of device for holding a canvas while they worked.
Then along came the seventeenth century and the rise of the Dutch Masters who were very astute and methodical in their working practices. They liked working with their canvases in a vertical position so they invented the three legged contraption.
This holding device bore some resemblance to the chief beast of burden, the donkey. After this, painters adopted the Dutch word for “little donkey” to their canvas bearing invention. This word sounded and looked like our modern word, easel.
Source:What’s in a Word by Webb Garrison