1560s, "immaterial," from un- (1) "not" + touchable (see touch (v.)). Meaning "that legally cannot be interfered with" is recorded from 1734. Meaning "too loathsome or defiling to be touched" is recorded from 1873. The noun, in reference to a hereditary low caste of India, is attested from 1909; the term and the restrictions were made illegal in India in 1947.
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