The phrase "metal rear" refers to someone's posterior (buttocks) being made of metal, which is not possible in reality. It is often used humorously to describe a person's strong or durable buttocks. Full definition
You may prefer this and the ability to remove the rear, while maintaining the lightweight design, than a heavier and potentially hotter metal rear of some competitors. (pocket-lint.com)
The 5X's metal rear also accounts for one of its major selling points. (androidcentral.com)
The shiny metal rear only helps to make you feel you've got a quality device in your hands. (carrypad.com)