I think samclem has it. According to my Concise OED, craps, referring to the game, started to be used in the 19th century, and came from crabs, “the lowest roll in dice.”
Well, they wouldn’t be saying that if it was the come-out roll and they were playing Don’t Pass. Assuming the casino or the game operator wasn’t barring 12, of course.
The game of craps is a very simple game to play, but the casino makes it seem complicated because of all the betting choices available on the table layout. The game also moves faster than any other, and between the table and the speed, newcomers can get confused quickly.
Start with the simplest (and safest) bets: Pass and Don’t Pass. Forget the rest, and forget odds bets on those two for now. Later, when you’re a little more comfortable with the speed of the game and knowledgeable about the bets, you might want to add odds bets on Pass/Don’t Pass, as well as Come/Don’t Come and Place numbers to your “repertoire” of bets.
All the others–the Field, the Propositions (that is, the Any 7, Any Craps, boxcars, hardways, and so on in the middle of the layout) are either sucker bets or have a far greater mathematical advantage to the casino than they should. Especially bad are the Big 6 and Big 8, which pay off at even money although you can get 6-to-5 on them if you play 6 and 8 as place numbers.
I don’t have the exact percentages handy, but there are any number of excellent references that will tell you just how much advantage the casino has at any given game (or in the case of craps, for any given bet). I’ve found Playboy’s Book of Games, by E. Silberstang and Scarne’s Complete Guide to Gambling by John Scarne, especially handy for explaining how the math works for the casino.