An Etymological Dictionary of the German Language/Tonne
Tonne, f., ‘tun, cask, barrel,’ from the equiv. MidHG. tunne, OHG. tunna, f. A corresponding word with a similar form is preserved by Du. ton, AS. tunne, E. tun; according to these LG. cognates the HG. word ought to have an initial z, or rather the cognates an initial d. Hence the word must have been borrowed by one or the other, which is all the more probable since it is wanting in Swab. and Bav. The Rom. languages have Fr. tonne (tonneau), Span. and Portug. tonel. Probably Kelt. is the ultimate source of the word; comp. Ir. and Gael. tunna, ‘tun, cask.’ In that case, the word passed into HG. after 700 A.D., When the t could not be permutated to z. The form of OSwed. þyn, ‘tun, cask,’ shows that it was borrowed at a much earlier period.