"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.
"patch of ground around a house," Old English geard "fenced enclosure, garden, court; residence, house," from Proto-Germanic *gardan- (source also of Old Norse garðr "enclosure, garden, yard;" Old Frisian garda, Dutch gaard, Old High German garto, German Garten "garden;" Gothic gards "house," garda "stall"), of uncertain origin, perhaps from PIE *ghor-to-, suffixed form of root *gher- (1) "to grasp, enclose," with derivatives meaning "enclosure."
As "college campus enclosed by the main buildings," 1630s. Shipyard is from c. 1700. In railway usage, "ground adjacent to a train station or terminus, used for switching or coupling trains," 1827. Yard sale is attested by 1976.
<a href="https://www.etymonline.com/word/backyard">Etymology of backyard by etymonline</a>
Harper, D. (n.d.). Etymology of backyard. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/backyard