I’ve been familiar with (and enjoyed) the big, meaty mushrooms called portobello for years, and I had assumed that that was, well, their name. But I just read the entry in the invaluable Food Lover’s Companion, which begins:
An extremely large, dark brown mushroom that is simply the fully mature form of the cremino, which in turn is a variation of the common cultivated white mushroom. The name “portobello” began to be used in the 1980s as a brilliant marketing ploy to popularize an unglamorous mushroom that, more often than not, had to be disposed of because growers couldn’t sell them.
Apparently it’s also called portabella; either way, the origin is unknown—and the word itself is still unknown to the OED. Surely the 1980s are recent enough that it should be possible to pinpoint the creation of the term?
Update. See now the Wordorigins thread on this topic, and the chapter excerpt linked from it; lots of interesting leads, still nothing definitive. How can an etymology be completely unrecoverable after only two decades?
Update (1/6/2007): See now Portabello redux, with the new OED entry and a suggested etymology.
I should add that there is an entry portobello in the OED, but it doesn’t seem mushroom-related:
? A kind of game resembling billiards.
1777 HOWARD Prisons Eng. 26 Gaming in various forms is very frequent: cards, dice, skittles, Missisippi and Portobello tables, billiards, fives, tennis, &c. Ibid. 198 One can scarcely ever enter the walls [of the King's Bench Prison] without seeing parties at skittles, missisippi, portobello, tennis, fives, &c.