On the Lam: Origin of a Gangster's Term

"'Slang,' as the poet Carl Sandburg has said, 'is a language that rolls up its sleeves, spits on its hands and goes to work,'" Professor Potts notes in Ball of Fire (1941). A particularly wizened sleeve-roller is the slang term "on the lam." Possibly of Old English derivation, the verb, "to lam" means "to beat." 

"On the lam" doesn't seem to be anywhere in modern conversation except in reference to vintage gangster movies where everybody spits it out like rounds from a Tommy gun. From the context you know it means someone has become a fugitive from the law and is in hiding, but from where did it originate?

Wanted gangster moll, Sugar Puss, holds up her tired tootsies after going on the lam. Ball of Fire (1941)

According to this William Safire article on the term, no one knows.  Since its original meaning refers to being beaten, someone who goes on the lam is trying to avoid a physical run-in with the authorities, notes Safire ("to avoid a feared lamming . . . one lams."). It would seem to me, though, (this is just a guess) that the beating could refer to one's foot hitting the road while fleeing, like the similar, more modern, alliterative term "to pound the pavement."

"Lam" today seems to survive mostly in the term "lambast," meaning to beat with a cane or to argue angrily. 

Just a note.