The term 'Underworld Lingo' refers to the language used by criminals and other people who are part of the underworld. The crossword solution for this term is 'CANT', which is an acronym for 'CANT, Argot, and New Tongue'. This term was coined in the 19th century to describe the secret language used by criminals and other members of the underworld. It is a combination of slang, jargon, and other words that are not commonly used in everyday language.