E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
According to a Welsh tradition, Edward I. promised to provide Wales with a prince who could speak no word of English, and when his son Edward of Carnarvon was born he presented him to the assembly, saying in Welsh Eich dyn (behold the man).
The more general belief is that it was the motto under the plume of John, King of Bohemia, slain by the Black Prince at Cressy in 1346, and that the Black Prince who slew the Bohemian assumed it out of modesty, to indicate that he served under the king his father.