A “mark” is someone who can easily be taken advantage of and came to us from midway carnival operators (or “carries”) who run games of chance.

The word midway was first used to describe the outdoor amusements at the 1893 World’s Exposition in Chicago.

After a camie found a victim, and before sending him on his way with a cheap prize, the rogue would slap the rube on the back with a dust-covered hand, marking him as a sucker for operators down the line.