1A person who comes from an aristocratic or privileged family background but has little or no inherited wealth.
In early use applied especially to younger sons of aristocratic families who received no inheritance under the system of primogeniture, and popularized by George Bernard Shaw.
2rare A person who pretends to be from a lower social background; an inverted snob.
Mid 19th century; earliest use found in Catherine Gore (d. 1861), novelist and playwright. From down- + start, after upstart.