late 13c., sugre, from Old French sucre "sugar" (12c.), from Medieval Latin succarum, from Arabic sukkar, from Persian shakar, from Sanskrit sharkara "ground or candied sugar," originally "grit, gravel" (cognate with Greek kroke "pebble"). The Arabic word also was borrowed in Italian (zucchero), Spanish (azucar), and German (Old High German zucura, German Zucker), and its forms are represented in most European languages (cf. Serb. cukar, Polish cukier, Russian sakhar).
Its Old World home was India (Alexander the Great's companions marveled at the "honey without bees") and it remained exotic in Europe until the Arabs began to cultivate it in Sicily and Spain; not until after the Crusades did it begin to rival honey as the West's sweetener. The Spaniards in the West Indies began raising sugar cane in 1506; first grown in Cuba 1523; first cultivated in Brazil 1532. The -g- in the English form cannot be accounted for. The pronunciation shift from s- to sh- is probably from the initial long vowel sound syu- (as in sure). Slang "euphemistic substitute for an imprecation" [OED] is attested from 1891. As a term of endearment, first recorded 1930. Sugar maple is from 1753. Sugar loaf was originally a moulded conical mass of refined sugar (early 15c.); they're now obsolete, but sense extended 17c. to hills, hats, etc. of that shape.