Caravaggio, Cardsharps (c. 1595)
"Voices may be concealed six wayes: First by absence, and this is the safest way, and if it be not discovered, it cannot be suspected. Then follows mumbling or low speaking, which is unseemly and full of suspition, and ofttimes is the cause of great mischiefs. The third is to speak in a forrain Tongue, as Greek, Latin, Germane, Italian; this also breeds suspition and is unseemly. The fourth is by nodding, as Men playing, but this is most ridiculous and unhandsome. The fift is by words that signifie other things... and this wants long observation: yet if one can do it handsomely there can be no suspition... The sixt is when we speak by cutting off some words, or pieces, this is not rediculous, and becomes a Grave man, because it makes a doubtfull sense, and it is so lawfull that it is familiar in the Writings of great Men."
Johann Jacob Wecker, Eighteen Books of the Secrets of Art and Nature (1660)Relaying secret messages is tough. Mumbling is unseemly, nodding is silly, and Secret German rarely works in Germany. Instead, just leave out a few syllables and words here and there. Your secret may be mangled, but bystanders will have to agree that you are... not ridiculous.