Old English sparian "to refrain from harming, be indulgent to, allow to go free; use sparingly," from the source of Old English spær "sparing, frugal," from Proto-Germanic *sparaz (source also of Old Saxon sparon, Old Frisian sparia, Old Norse spara, Dutch sparen, Old High German sparon, German sparen "to spare"). Meaning "to dispense from one's own stock, give or yield up," is recorded from early 13c. Related: Spared; sparing.
"kept in reserve, not used, provided or held for extra need," late 14c., from or from the same root as spare (v.). Old English had spær "sparing, frugal." Also compare Old Norse sparr "(to be) spared." In reference to time, from mid-15c.; sense of "lacking in substance; lean, gaunt; flimsy, thin; poor," is recorded from 1540s. Spare part is attested from 1888. Spare tire is from 1894 of bicycles; 1903 of automobiles; 1961 of waistlines.
"extra thing or part," 1640s, from spare (adj.). The Middle English noun sense was "a sparing, mercy, leniency" (early 14c.). Bowling game sense of "an advantage gained by a knocking down of all pins in two bowls" is attested from 1843, American English.