The cant language of thieves and gypsies, called Pedlars’ French, and St. Giles’s Greek: see ST. GILES’S GREEK. Also the mystic language of Geber, used by chymists. Gibberish likewise means a sort of disguised language, formed by inserting any consonant between each syllable of an English word; in which case it is called the gibberish of the letter inserted: if F, it is the F gibberish; if G, the G gibberish; as in the sentence How do you do? Howg dog youg dog.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Gib Cat * GibbeNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes