Thieves can't was a secret language used by thieves, beggars and hustlers in the United Kingdom. It originated around the 1530's, and was created so the thievery and secrets of the people could be kept private to them. According to a myth, it was created by the King of the Gypsies and Cock Lorel in a cave in Derbyshire,  for that exact reason. 

It was used on flyers, pamphlets and rogue literature in the Elizabethan period. 

Examples of words are:

Glymmer for fire,

Bene for good,

Cove for man,

Mort for woman and

Lag for water.