I am pretty sure the origin is indeded from Craps just as you found. A come is an even money bet (ie bet $1 and you receive $1 if you win).
In Craps, the only real difference between the Pass Line and the Come Bet is that you make a come bet after the point has been established on the Pass Line. After you make a come bet, the first roll of the dice will establish the Come point. If a 7 or 11 rolls, you are an automatic winner. But, if you roll 2, 3 or 12 on the first roll you lose.
So Seven Come Eleven, means you that any time after the first roll when a shooter has a point to make You win on natural seven or eleven and lose on craps (two, three or twelve). Any number that comes up is a “come point”, and must be thrown before a seven is thrown.
Check out the sources for some explanation of craps rules, if you are interested.