Arrow-heads of the neolithic period. The shafts of these arrows were reeds, and the heads were pieces of flint, carefully sharpened, and so adjusted as to detach themselves from the shaft and remain in the wounded body. At one time they were supposed to be shot by elves at people and cattle out of malice or revenge.
There every herd by sad experience knows How, winged with fate, their elf-shot arrows fly, When the sick ewe her summer food forgoes, Or stretched on earth the heart-smit heifers lie.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894