late 14c., "exempt from service or obligation," from Old French immunité and directly from Latin immunitatem (nominative immunitas) "exemption from performing public service or charge," from immunis "exempt, free," 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" (see mutable). Medical sense "protection from disease" is 1879, from French or German.