How did smacker or smackeroo come to mean a dollar? 10 simoleans to anyone who can figure it out. That’s a nice chunk of cheddar.
It’s from smacking your money down on the bar for a drink or when betting at a gambling table.
Awww. 
to you too!
I’ve heard smack, but not smacker.
And a much more satisfying sound when you were smacking a silver dollar instead of a piece of paper.
Etymology Online gives a slightly different story, qualified with “perhaps”:
“money,” c. 1918, American English slang, perhaps from smack (v.2) on notion of something “smacked” into the palm of the hand
I think I’ve heard it used of £££.
Yes, the word seems to have travelled across the pond. The OED says
slang (originally U.S.). A coin or note of money; spec. a dollar; a pound.
I like this one. For some reason I didn’t notice it when searching. It reminds me of an old Rosie O’Donnell comedy bit where she exclaims, “Jennifer - smack my palm!”
Other way around for me, but most usually “smackeroos.” “500 smakckers” sounds fine. “509 smacks” apparently is not part of my dialect. (Though all are old-timey sounding.)