quatre-vingts

…it survives in the word quatre-vingts (“four twenties”), for 80; other traces are found in ancient Celtic, Gaelic, Danish, and Welsh. The base 60 still occurs in measurement of time and angles.

…(5) shows the use of quatre-vingts for 80, commonly found in French manuscripts until the 17th century and occasionally later, the numbers often being written like iiij^xx, vij^xx, and so on; and (6) represents the coefficient method, “four C” meaning 400, a method often leading to forms like ijM or…