The meat is the whole abdomen of a radroach. Since radroaches appear less frequently in Fallout: New Vegas than in Fallout 3, radroach meat is more difficult to find. The effects of radroach meat in Fallout: New Vegas are notably different: It restores a small amount of hit points, but gives a high dose of radiation, in addition to a -1 penalty to Strength.
Within the game files exists a version of radroach meat, that gives 18 rads instead of the normal 10. It was supposed to be dropped by the irradiated radroach, but by cutting the creature from the game, this variant of the meat was cut as well.