Innovative Language French Lesson 13 dixit:
The verb arnaquer ("to rip off") derives from the slang harnacher, meaning "to entertain with the intention of swindling." Then the word lost its first letter, h-, in the nineteenth century, and the French used the word to mean "to cheat." In 1900, the spelling of this word evolved to its present form, but its definition shifted to mean "fixing a game."
You will certainly hear the phrase C'est l'arnaque ("It's a rip-off") in many mouths in France, too.