To be branded with a K (kalumnia). So, according to the Lex Memmia, false accusers were branded in the forehead.
The three bad K's. The Greeks so called the Karians, Kretans, and Kilikians. The Romans retained the same expression, though they spelt the three nations with C instead of K.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894