1936, derogatory slang form of pink (n.), used of people whose social or political views "have a tendency toward 'red;' " a metaphor that had existed since at least 1837. As an adjective by 1957.
: How come those Niggers and those creepy pinko hippies call us names like ''pigs''? (1957+)
A person of liberal or mildly radical socialist political opinions; pink: Nixon liked to call people pinkos (1936+)