mid-15c., "free from mental or moral pollution, pure," from a figurative use of Latin immaculatus "unstained," from assimilated form of in- "not, opposite of" (see in- (1)) + maculatus "spotted, defiled," past participle of maculare "to spot," from macula "spot, blemish," a word of uncertain origin. The literal sense of "spotlessly clean or neat" in English is first attested 1735. Related: Immaculately.
The phrase Immaculate Conception "freedom from original sin possessed by the Virgin Mary from her conception in her mother's womb" is from late 15c. in English (from French conception immaculée); the idea itself had been debated in the Church since 12c., declared to be an article of faith in 1854.