Why is the ninth month called September?
1640s, "one who schemes;" agent noun from design (v.). Meaning "one who makes an artistic design or a construction plan" is from 1660s. In fashion, as an adjective, "bearing the label of a famous clothing designer" (thus presumed to be expensive or prestigious), from 1966. Designer drug attested from 1983.
Of high quality; bearing a famous label: Ah, designer ennui (1960s+)noun
A counterfeiter (1940s+ Underworld)