1570s, common name of Dianthus, a garden plant of various colors, of unknown origin. Its use for "pale rose color" first recorded 1733 (pink-coloured is recorded from 1680s), from one of the colors of the flowers. The plant name is perhaps from pink (v.) via notion of "perforated" petals, or from Dutch pink "small" (see pinkie), from 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 pale red flowers.
The flower meaning led (by 1590s) to a figurative use for "the flower" or finest example of anything (e.g. Mercutio's "Nay, I am the very pinck of curtesie," Rom. & Jul. II.iv.61). 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 first recorded 1915. To see pink elephants "hallucinate from alcoholism" first recorded 1913 in Jack London's "John Barleycorn."