adj. Relating to a device that can be operated using one or both thumbs.
The PDA is equipped with the same thumbable QWERTY keyboard and removable flip lid as the original Treo 180.
—Bruce & Marge Brown, “Featherweight Organizer from Handspring,” PC Magazine, July 01, 2002
1997 (earliest)
[At] the press event, the two execs delineated a number of features that will set the new Series 5 apart from previous incarnations. Instead of being merely "thumbable," the new cushioned keypad on the Series 5 "allow you to type with speed and accuracy," according to Myers.
—Jacqueline Emigh, “Psion Launches Series 5 Handheld In US,” Newsbytes, June 17, 1997
There may be a few issues that Jeff Cox and the editors of Rodale Press Garden Books don't cover in "Your Organic Garden," the companion volume to their new PBS series, but most gardeners will be hard-pressed to find them. . . . [It's] a fine companion — the perfect thumbable resource to turn to after Cox has shown you how easy — and what fun — organic gardening can be.
—M. A. Mariner, “Organically speaking,” The San Francisco Chronicle, January 19, 1994