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 Middle French conception immaculée); the idea itself had been debated in the Church since 12c., declared to be an article of faith in 1854.