backstop n.

"being behind, away from the front, in a backward direction," Middle English, from back (n.) and back (adv.); it is often difficult to distinguish from these when the word is used in combinations. Formerly with comparative backer (c. 1400), also backermore. To be on the back burner in the figurative sense of "postponed" is by 1960, from the image of a cook keeping a pot there to simmer while at work on another concoction at the front of the stove.

late 14c., "a plug;" mid-15c., "a cessation," from stop (v.). Of mechanisms of musical instruments from c. 1500, especially of organs, where opening them makes it produce more sound, hence figurative phrase pull out the stops (1909). From 1660s in phonetics, 1831 in photography. Meaning "a stopping place" is from 1889. To put a stop to some activity is from 1670s (earlier give a stop to, 1580s).