late 14c., "a spot or stain of ink;" also "a moral stain or blemish, a disgrace, a sin;" a word of uncertain origin. Perhaps from Old Norse blettr "blot, stain," or from Old French blot, variant of bloc "block." The Middle English Compendium compares, hesitantly, Old French blo(s)tre, variant of blestre "a boil." It is attested from 1570s as "any black or dark patch."
early 15c., "to make blots (with ink), disfigure with blots," also figurative; mid-15c. "to blot out, obliterate" (words), from blot (n.). Related: Blotted; blotting.
updated on October 17, 2022