Terminology
While a person is being sought for potential arrest, the person may be described variously as being "at large," "considered armed and dangerous," or as a "person of interest" to law enforcement. The latter term is frequently used in an "All-points bulletin" issued to other law enforcement persons or agencies. A person who has jumped bail after arraignment in court may be hunted or pursued by his bail bondsman, and a bounty may be "on his head." The act of fleeing from the jurisdiction of a court is described colloquially as "fleeing justice" or "running from the Law." A "wanted poster" may be issued, especially by the FBI, culminating in the "FBI's Most Wanted List" of fugitives.
"On the lam" or "on the run" often refers to fugitives. Mencken's The American Language and The Thesaurus of American Slang proclaim that lam, lamister, and "on the lam" — all referring to a hasty departure — were common in thieves' slang before the turn of the 19th to 20th century. Mencken quotes a newspaper report on the origin of 'lam' which actually traces it indirectly back to Shakespeare's time.
Its origin should be obvious to anyone who runs over several colloquial phrases for leavetaking, such as 'beat it' and 'hit the trail'. The allusion in 'lam' is to 'beat,' and 'beat it' is Old English, meaning 'to leave.' During the period of George Ade's 'Fables in Slang' (1900), cabaret society delight in talking slang, and 'lam' was current. Like many other terms, it went under in the flood of new usages of those days, but was preserved in criminal slang. A quarter of a century later it reappeared.Mencken also quotes a story from the New York Herald Tribune in 1938 which reported that "one of the oldest police officers in New York said that he had heard 'on the lam' thirty years ago."
Read more about this topic: Fugitive