(Anglo-Saxon, ændlefene, aend = ain, lefene = lef, left). One left or one more after counting ten (the fingers of the two hands). Twelve is Twa lef (two left); all the other teens up to 20 represent 3, 4, 5, etc. + ten. It would seem that at one time persons did not count higher than twelve, but in a more advanced state they required higher numbers, and introduced the “teen” series, omitting eleven and twelve, which would be enteen and twateen.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894