1560s, to remove (a nut, etc.) from a shell, from shell (n.). The meaning to bombard with shells is first attested 1856. To shell out disburse (1801) is a figurative use from the image of extracting nuts. Related: Shelled; shelling.
There are 14 calories in 1 cup, slices (3 oz) of Nopales (Cactus Leaf), raw. You'd need to walk 4 minutes to burn 14 calories. Visit CalorieKing to see calorie count and nutrient data for all portion sizes.
Farmed nopales are most often of the species Opuntia ficus-indica, although the pads of almost all Opuntia species are edible. The other part of the nopal cactus that is edible is the fruit called the tuna in Spanish, and the prickly pear in English.