mid-15c., "free, exempt" (from taxes, tithes, sin, etc.), from Latin immunis "exempt from public service, untaxed; unburdened, not tributary," literally ""not paying a share," from assimilated form of in- "not, opposite of" (see in- (1)) + munis "performing services" (compare municipal), from PIE *moi-n-es-, suffixed form of root *mei- (1) "to change, go, move." Specific modern medical sense of "exempt (from a disease)," typically because of inoculation, is from 1881, a back-formation from immunity. Immune system attested by 1917.
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