prog

1530s, "to poke with a stick," of uncertain origin; possibly [Barnhart, Century Dictionary] a variant of brod, from Middle English brodden "to goad," from Old Norse broddr "shaft, spike" (see brad), or perhaps imitative [OED]. Compare dialectal prog "pointed instrument for poking" (1610s), also as a verb, "to poke about." 

Figurative sense of "mental incitement or instigation" is by 1871. Related: Prodded; prodding.

c. 1600, "characterized by advancement, going forward, moving onward" (in action, character, etc.), from progress (n.) + -ive, or else from French progressif, from past participle stem of Latin progredi. Specifically of taxation, from 1889. From the notion of "using one's efforts toward advancement or improvement" comes the meaning "characterized by striving for change and innovation, avant-garde, liberal" (in arts, etc.), from 1908; of jazz, from 1947.

In the socio-political sense "favoring reform; radically liberal" it emerged in various British contexts from the 1880s; in the U.S. it was given to a movement active in the 1890s and a generation thereafter, the name being taken again from time to time, most recently by some more liberal Democrats and other social activists, by c. 2000.

The noun in the sense "one who favors, promotes, or commends social and political change in the name of progress" is attested by 1865 (originally in Christianity). Earlier in a like sense were progressionist (1849, adjective; 1884, noun), progressist (1848). Related: Progressively; progressiveness.