E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A corruption of fingers, that is, digits (Latin, digiti, fingers). So called from the primitive method of marking the monades by the fingers. Thus the first four were simply i, ii, iii, iiii; five was the outline of the hand simplified into a v; the next four figures were the two combined, thus, vi, vii, viii, viiii; and ten was a double v, thus, x. At a later period iiii and viiii were expressed by one less than five (i-v) and one less than ten (i-x). Nineteen was ten-plus-nine (x + ix), etc.a most clumsy and unphilosophical device.