arpeggione

from The Century Dictionary.

• noun A small six-stringed violoncello, invented by Staufer of Vienna in 1823, having a body shaped somewhat like that of a guitar: hence also called guitar-violoncello.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A six-stringed musical instrument of the nineteenth century, fretted and tuned like a guitar but bowed like a cello.