Do you know ghouls from goblins and ghosts?
late 14c., "cloudy, misty," from Latin nebulosus "cloudy, misty, foggy, full of vapor," from nebula (see nebula). The figurative sense of "hazy, vague, formless" is first attested 1831. Astronomical sense is from 1670s. Related: Nebulously; nebulousness.