The story
From the lack of definition (fuzziness) due to the wave-like nature of fuzzy dark matter, unlike the particle-like of hot dark matter and cold dark matter.
Etymology from Wiktionary, CC BY-SA 4.0.
fuzzy dark matter
Today it means: A type of dark matter with very low particle mass, with concomitant very large de Broglie wavelength (such as wavelength on the order of kiloparsecs), resulting in a population of overlapping waves, and concentrations of this dark matter modeled as fields and interference patterns, over astronomical and cosmological distances. Full definition →