What is the origin of the term "Couch Potato"?

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?

Solution 1:

Couch Potato was introduced by Robert D. Armstrong in the book The Official Couch Potato Handbook

From http://classic-web.archive.org/web/20080521102001/http://www.potatomuseum.com/exCouch.html

"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.

Solution 2:

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".