Old English wyrd (n.) "fate, destiny," literally "that which comes," from Proto-Germanic *wurthis (cf. Old Saxon wurd, Old High German wurt "fate," Old Norse urðr "fate, one of the three Norns"), from PIE *wert- "to turn, wind," (cf. German werden, Old English weorðan "to become"), from root *wer- (3) "to turn, bend" (see versus). For sense development from "turning" to "becoming," cf. phrase turn into "become."
The modern sense of weird developed from Middle English use of weird sisters for the three fates or Norns (in Germanic mythology), the goddesses who controlled human destiny. They were portrayed as odd or frightening in appearance, as in "Macbeth," which led to the adjectival meaning "odd-looking, uncanny," first recorded 1815.