E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Connected with must. In 1382 Philip the Bold, Duke of Burgundy, granted to the town of Dijon, noted for its mustard, armorial bearings with the motto MOULT ME TARDE (Multum ardeo, I ardently desire). The arms and motto, engraved on the principal gate, were adopted as a trade-mark by the mustard merchants, and got shortened into Moult-tarde (to burn much).
The nasturtium is of the mustard family, in Spanish masturcio; and the Italian mustarda is mustard.