Well, we Americans have some unusual expressions as I'm sure you are aware. Gun violence isn't a recent phenomena. Have you ever heard the expression "He's going postal" or "He's gone postal" ? There was a period during the late 1980's to mid 1990's in which a number of postal workers came into work with a gun and started shooting their bosses and co-workers. So, the expression was born, and people use the phrase to refer to someone who has gone completely berserk.
Most of our slang is borrowed. Originally it came from the UK, then Hollywood brought us Americanisms, a couple of world wars added a few more, but the rest have just squeezed into the language as happens everywhere.
"Blind Freddy could have seen that" is usually hurled at unpopular refereeing decisions. Blind Freddy was a blind man who drank at the Bat and Ball Hotel, near the Sydney Cricket Ground.
"He shot through like a Bondi tram" just means he absquatulated. The trams ran from the CBD to Bondi until the late 1950s.
"You've got two chances: none and Buckley's." Buckley was the name of a bank manager murdered by Squizzy Taylor in the (I think) 1920s.
But one word we've taken, and beaten, and moulded into something unique is bâ–²stard. It has so many shades of meaning -- the majority complimentary -- that it deserves a dissertation all by itself.
Harry 