n. An error made while using the thumbs to type, particularly on a mobile device keypad.
2010
How do you get rid of "thumbos," those cellphone message typos? Practice. And some handy tips.
2010
Sent from my Palm Pre. Forgive thumbos.
2009 (earliest)
thumbo. A typographical error made while using thumbs only to text mesage [sic].
I didn't notice the "typo" ("thumbo?") in the lower right corner of the first page until i scrolled down to the second page.