“Green thumb” is used to describe someone’s skill at gardening or growing plants.
“Green fingers” first appeared in the 1930s, followed about ten years later by “green thumb.” As to how one’s thumb or fingers get green, there seem to be several theories.
It may come from the fact that algae growing on the outside of earthenware pots will stain a person’s thumb if he or she handles enough pots. Hence, a person who is always working with flowerpots has a green thumb.
Another theory is that it originated during the reign of King Edward I of England. He was fond of green peas and kept half a dozen serfs shelling them during the season. The serf who had the greenest thumb won a prize. Being speedy at shelling probably kept him from eating too many of those scrumptious little morsels.
When good gardeners eliminate unwanted shoots from a stem by pinching them between the thumbnail and the index finger, the thumb becomes green near the nail.
The eminent etymologist Eric Partridge suggested that “green thumb” referred to the very old English proverb, “An honest miller has a golden thumb.” Millers, who grind corn for farmers, used to judge the quality of their corn flour by rubbing a bit between the palm and thumb. But millers were often suspected of cheating their customers, and “golden thumb” was often used sarcastically, to mean a talent for duplicity. The proverb was well enough known in Britain in the mid-1900s to make the “golden thumb” and “green thumb” connection plausible.
The earliest Oxford English Dictionary citation for “green fingers” is from The Misses Make-Believe, a 1906 novel by the Scottish-born writer Mary Stuart Boyd. The first Oxford citation for “green-thumbed” is from the June 6, 1937, Washington Post: “He is, I think, the ‘green-thumbed’ type of gardener, who has lived and loved his flowers and has learned from them and from the soil.”
Now, I happen to have a black thumb. The Merriam-Webster Dictionary defines “black thumb” as a notable inability to make plants grow; a tendency to fail as a gardener.
That’s me. I beg people not to give me plants because they wilt at once, committing suicide rather than suffering a slow death from lack of water or sunshine.