Slangs are the greaves with which the legs of convicts are fettered; hence convicts themselves; and slang is the language of convicts.
The difficulty of tracing the fons et origo of slang words is extremely great, as there is no law to guide one. Generally, a perversion and a pun may be looked for, as Monsengueur = toe (q.v.), Monpensier = ventre (i.e. monpanse, my paunch or belly), etc. (See Sandis,Squash, and numerous other examples in this dictionary. For ryming slang see Chivy.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894