c. 1400, "having power to control fate," from wierd (n.), from Old English wyrd "fate, chance, fortune; destiny; the Fates," literally "that which comes," from Proto-Germanic *wurthiz (source also of Old Saxon wurd, Old High German wurt "fate," Old Norse urðr "fate, one of the three Norns"), from PIE *wert- "to turn, to wind," (source also of German werden, Old English weorðan "to become"), from root *wer- (2) "to turn, bend." For sense development from "turning" to "becoming," compare phrase turn into "become."
The sense "uncanny, supernatural" 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" (and especially in 18th and 19th century productions of it), which led to the adjectival meaning "odd-looking, uncanny" (1815); "odd, strange, disturbingly different" (1820). Related: Weirdly; weirdness.