Of American-English origin, the informal term couch potato denotes somebody who sits around vegetating in front of the television—cf. also the phrase bums on seats, of a similar form but of a very different meaning.

The earliest instance of couch potato that I have found is from the Los Angeles Times (Los Angeles, California) of Friday 28^th December 1979, which published an article titled Doo Dah: Pasadena’s Pleasure Parade, by Sam Kaplan, about the Doo Dah Parade, a festive alternative to the Tournament of Roses Parade:

Entries this year include:
—The All-City Waitresses Marching Band, with about 100 women in white uniforms and aprons performing with pots, pans, spoons, eggbeaters and other kitchen utensils.
[…]
—The Humboldt State Marching Lumberjacks, an out-of-state entry, and the Couch Potatoes, who will be lying on couches watching television as they are towed along the parade route.

Several, not mutually exclusive, reasons explain the choice of potato in couch potato:
– association of potato with vegetable in the sense of somebody who leads an uneventful life, without intellectual or social activity;
– potato, which conjures up the image of plumpness, connotes unfitness and overweight from lack of physical exercise;
– potato may be a pun on the second element of boob tuber, a compound denoting somebody who spends a lot of time watching television—the pun being that the potato is a tuber.

The compound boob tuber is derived from boob tube, which denotes both television and a television set, and in which boob means foolish, stupid, as in this is a boob thing to do.

The earliest instance of boob tube that I have found is from the last television column that William Ewald (1925-2007) wrote for United Press International, published in many US newspapers on Friday 14^th August 1959—for example, in The Lawton Constitution (Lawton, Oklahoma):

‘TV Landsliding Us With Garbage’
‘Boob-Tube’ Critic Quits
New York (UPI)—Dear reader: This is my last gasp as a TV columnist.
[…]
As a chronicler of the boob-tube, I have received hundreds of letters.

The form boob toob has also been used, as in the following from the column Best of Hollywood, by Mike Connolly, in The Philadelphia Inquirer (Philadelphia, Pennsylvania) of Thursday 12^th October 1961:

Charlton Heston* calls his TV set “The Boob Toob.”

* Charlton Heston (1923-2008), American actor and social activist

The earliest use of boob tuber that I have found is from a letter by one Mrs. M. A. Diehr, published in The Indianapolis Star (Indianapolis, Indiana) of Saturday 15^th December 1962:

I think I knew the first time I saw “It’s A Man’s World” that no television network in its right mind would continue the series. It was a well-thought-out and well-acted show. The first such series, needless to say, I’d had the pleasure of watching.
And consequently, if I liked it, it was doomed. It didn’t achieve the standard of mediocrity guaranteed to mesmerize the network’s quota of “boob-tubers.”