thumbo
n. An error made while using the thumbs to type, particularly on a mobile device keypad. [Blend of thumb and typo.]

Example Citations:
How do you get rid of "thumbos," those cellphone message typos? Practice. And some handy tips.
—"Touch Type With Your Thumbs," One News Page, March 17, 2010

Sent from my Palm Pre. Forgive thumbos.
—Jennifer 8. Lee, jenny8lee.com, Posterous, March 17, 2010

Earliest Citation:
thumbo. A typographical error made while using thumbs only to text mesage [sic].
—"thumbo," Urban Dictionary, October 3, 2009

Notes:
Here's an earlier instance of the word, although in this case the writer is referring to an image of a thumb visible in a scanned document:

I didn't notice the "typo" ("thumbo?") in the lower right corner of the first page until i scrolled down to the second page.
—rtdunham, "Google books iPhone-friendly; Amazon Kindle books next?," AppleInsider, February 7, 2009

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