1889, civvies, short for civilian clothes (see civilian); in reference to civilian clothes of military men.
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civilian n.
late 14c., "judge or authority on civil law," from noun use of Old French civilien "of the civil law," created from Latin civilis "relating to a citizen, relating to public life, befitting a citizen; popular, affable, courteous," alternative adjectival derivative of civis "townsman" (see city). Sense of "non-military and non-clerical person, one whose pursuits are those of civilian life" is attested by 1766. As an adjective, "pertaining to or characteristic of a civilian," from 1640s.