I know the meaning of "couch potato" is a person living a mostly sedentary lifestyle who likes to watch TV while lying on the couch, but why potato?
Couch Potato was introduced by Robert D. Armstrong in the book The Official Couch Potato Handbook
"Very few words have a birthday so precise, and so precisely known, as couch potato. It was on July 15, 1976, we are told, that couch potato came into being, uttered by Tom Iacino of Pasadena, California, during a telephone conversation. He was a member of a Southern California group humorously opposing the fads of exercise and healthy diet in favor of vegetating before the TV and eating junk food (1973). Because their lives centered on television--the boob tube (1966)--they called themselves boob tubers. Iacino apparently took the brilliant next step and substituted potato as a synonym for tuber. Thinking of where that potato sits to watch the tube, he came up with couch potato.
Etymonline says the usage was first recorded in 1979.
Google NGrams apparently backs this up, and yet shows an anomalous blip around the turn of the 20th century:
What to make of the 1900 bulge, I can't say.
"Potato" is used presumably because it's inert, shapeless and plump: like most people who spend their lives in front of the TV. It also has a similar sound to other expressions using a stressed modifier followed by "potato": hot potato, sweet potato, mashed potatoes, etc., so there was a sound to mimic to make it sound "right".
The Oxford English Dictionary says:
couch potato n. [potato apparently punningly after (boob-)tuber = television addict and vegetable tuber, but perhaps simply by association with vegetable n. 2] slang (orig. U.S.) a person who spends leisure time passively or idly sitting around, esp. watching television or videotapes.
1979 Los Angeles Times (California Record ed.) 28 Dec. iv. 1/6 The Humboldt State Marching Lumberjacks..and the Couch Potatoes who will be lying on couches watching television as they are towed toward the parade route.
The New Partridge Dictionary of Slang and Unconventional English (2008) says:
couch potato noun a person who habitually idles, watching television. Possibly a pun on ‘boob-tuber’ (a television addict) and a ‘potato’ as a ‘tuber’; it may also play on VEGETABLE (a person with an undemanding existence); the ‘couch’, of course, is where the potato is planted. One of the very few slang words or phrases where it is seemingly possible to trace the coining; in July 1976 a group of friends in California coined the term, which was first used in commerce in 1977 and then hit the big time with the Official Couch Potato Handbook (1983) US, 1976
Here's the origin according to posts made on the American Dialect Society mailing list.
First from Barry Popik:
OED has "couch potato" from 1979.
USPTO records show it was trademarked by Robert Armstrong of Dixon, CA, with first use of July 15, 1976, and first use in commerce of April 20, 1977.
Larry Horn concurred:
Robert was a fellow-traveller of R. Crumb and his Cheap Suit Serenaders, a somewhat funky string band. They all holed up in a nice country house in Sonoma County at the time, and Robert lived off the royalties of "couch potato" and silk-screened T-shirts for a while back then. Some of the T-shirts featured a laid-back spud qua couch potato, i.e. a "tuber". Them was the days.
Finally from A. Allan:
The origin of "couch potato" is explained in detail in a book to be published in October by Houghton Mifflin:
"Predicting New Words: The Secrets of Their Success" by Allan Metcalf. Here is a pertinent excerpt:
. . . Here's how it chanced to happen, according to an illustrated history by two of the perpetrators themselves, "Elders" Jack Mingo and Robert Armstrong, in The Official Couch Potato Handbook (1983). The saga began in the 1960s with nine Southern Californians who got together on Thursday nights to watch Lost in Space. Calling themselves the "Lost in Space Club," they soon began meeting to watch other television shows as well. "One of them," the story continues, "known only as 'The Hallidonian,' soon made the discovery that any day, any time was all right for prolonged, indiscriminate TV viewing."
Then, supposedly on July 15, 1976, another of the nine "Elders," Tom Iacino, uttered the term couch potato in making a phone call to The Hallidonian. The illustrated history depicts the moment: "Hi, Annie Jo--Can I speak to the 'couch potato'?" asks Iacino's telephone voice, to which Annie Jo responds "The wha?" while across the room the Hallidonian relaxes on his couch, watching The Flintstones. . .
It might have something to do with people being called a 'vegetable' when they are unconscious in a coma (in a vegetative state).
Vegetables are inanimate objects. A potato is a vegetable, chosen for the phrase perhaps because of it's plump shape that would be associated with some who doesn't do any exercise and would likely be overweight.
Philo Farnsworth... is credited for inventing the television and once lived in Idaho which is famous for its potatoes. People who watch lots of television are often called couch potatoes. There could be some correlation between the term and the origin of the television.
If you look at the MyOldRadio.com website, episode 304 of the Our Miss Brooks radio show (which aired August 28, 1955) is entitled "Couch Potato." I assume this is the actual title of the episode that the people who run the website got off of some sort of log for the show; but we all know what happens when one assumes something.