What does on the lam mean?

Answer 1

First was actually British slang and later became American slang.

Probably related to the Old Norse word "lemja."

Lemja: to cripple by beating, to flog, to beat, to thrash,

also; to escape, flee, suggests hasty departure, etc.

But wait, there's more: It was first attributed to being American slang in two books one entitled Mencken's 'American Language' and the other, the 'Thesaurus of American Slang' by Berry and Van den Bark. There was a newspaper report Menken quoted that was on the origins of "on the lam" that actually traces it indirectly back to the late 1500's--early 1600's, the time of Queen Elizabeth I and Shakespeare: '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.'

So by the account of the newspaper it isn't attributed directly to Old Norse but Old English. It's clear that it is Old Norse, though, because "to beat it" is exactly what the word Lemja means! In fact even though "on the lam" has certainly gone out of fashion in America, we say and hear "beat it" all the time, still! So we can attribute it to both. It was British slang first.

Answer 2

"On the lam" means fleeing from something, usually the police. "Lam" as a verb means to flee hastily, to make a quick escape, although the word is almost never used as a verb.