also boot-strap, tab or loop at the back of the top of a men's boot, which the wearer hooked a finger through to pull the boots on, 1870, from boot (n.1) + strap (n.).
Circa 1900, to pull (oneself) up by (one's) bootstraps was used figuratively of an impossible task (among the "practical questions" at the end of chapter one of Steele's "Popular Physics" schoolbook (1888) is, "30. Why can not a man lift himself by pulling up on his boot-straps?" and an excellent question it is). By 1916 the meaning of the phrase had expanded to include "better oneself by rigorous, unaided effort." The meaning "fixed sequence of instructions to load the operating system of a computer" (1953) is from the notion of the first-loaded program pulling itself (and the rest) up by the bootstrap. It was used earlier of electrical circuits (1946).