Bionic arms, legs, eyes and so on are electronic body parts, built to work like living ones — and in stories, to be stronger or faster than the real thing. Engineers do make real electronic limbs that help people after injury; the everyday adjective then is a bit admiring, a bit science-fiction. In casual talk, though, people often use bionic with a smile: you ran up the stairs like you’re bionic, meaning only that you seemed unnaturally strong or quick. That joke-sense is very common. The word is informal when it is a joke, more technical when it names a device. Though actually the word itself is a blend of bio- and electronic, and the super-strong-body meaning spread from 1970s American television — The Six Million Dollar Man and The Bionic Woman — which is why it still sounds a little like a show.