old name for mercurous chloride, 1670s, from French calomel, supposedly (Littré) from Greek kalos "fair" + melas "black;" but as the powder is yellowish-white this seems difficult. "It is perhaps of significance that the salt is blackened by ammonia and alkalis" [Flood].
calomel cal·o·mel (kāl'ə-měl', -məl)
A colorless, white or brown tasteless compound used as a purgative and an insecticide. Also called mercurous chloride.