(THUM.uh.bul; th as in thin) adj. Relating to a device that can be operated using one or both thumbs.

Example Citation:
The PDA is equipped with the same thumbable QWERTY keyboard and removable flip lid as the original Treo 180.
—Bruce and Marge Brown, "Featherweight Organizer from Handspring," PC Magazine, July, 2002

Earliest Citation:
[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

Thumb as a verb — meaning to turn the pages of a book or magazine using (or as though using) one's thumb — has been around since the 1930s. (The OED's earliest citation comes from Dashiell Hammett's The Maltese Falcon, in 1930.) So it's a bit surprising that a book/magazine sense of the adjective thumbable (meaning something like "compulsively glance-throughable") didn't turn up until fairly recently:

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

After posting this term, reader Ken Gillis wrote in about a different sense:

As an aside to your logophilia item "thumbable", the term has a long history of use in the automotive industry, referring to sealants having the general consistency of modeling clay, Mortite, "Dum-dum" etc., which are applied (very rapidly, by experienced workers) with pressure from the thumb to openings in the coachwork to prevent the wind from whistling through until after the vehicle has left the showroom. The term is often used quite formally, as in the titles of technical specifications for such materials, and has been current for at least the last 30 years and is still in general use.

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