1936, derogatory slang form of pink (adj.) in the political sense, used of people whose social or political views "have a tendency toward 'red;' " a metaphor that had existed at least since 1837. As an adjective by 1957.
Entries linking to pinko
1570s, common name of Dianthus, a garden plant of various colors; a word of unknown origin. It is perhaps from pink (v.) via the notion of "perforated" (scalloped) petals. Or perhaps it is from Dutch pink "small, narrow" (see pinkie), itself obscure, via the term pinck oogen "half-closed eyes," literally "small eyes," which was borrowed into English (1570s) and may have been used as a name for Dianthus, which sometimes has small dots resembling eyes.
The noun meaning "pale red color, red color of low chroma but high luminosity" is recorded by 1733 (pink-coloured is recorded from 1680s), from one of the common colors of the flowers. The adjective pink is attested by 1720. As an earlier name for such a color English had incarnation "flesh-color" (mid-14c.), and as an adjective incarnate (1530s), from Latin words for "flesh" (see incarnation) but these also had other associations and tended to drift in sense from "flesh-color, blush-color" toward "crimson, blood color."
The flower meaning led (by 1590s) to a figurative use for "the flower" or highest type or example of excellence of anything (as in Mercutio's "Nay, I am the very pinck of curtesie," Rom. & Jul. II.iv.61). Compare flour (n.). The political noun sense "person perceived as left of center but not entirely radical (i.e. red)" is attested by 1927, but the image dates to at least 1837. Pink slip "discharge notice" is attested by 1915; pink slips had various connotations in employment in the first decade of the 20th century, including a paper signed by a worker to testify he would leave the labor union or else be fired. To see pink elephants "hallucinate from alcoholism" is from 1913 in Jack London's "John Barleycorn."