late 14c., "cloudy, misty," from Latin nebulosus "cloudy, misty, foggy, full of vapor," from nebula "mist, vapor" (from PIE root *nebh- "cloud"). The figurative sense of "hazy, vague, formless" is first attested 1831. Astronomical sense is from 1670s. Related: Nebulously; nebulousness.
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