mid-15c., "engage in rivalry, compete," from Old French contendre and directly from Latin contendere "to stretch out; to shoot, hurl, throw; strive after mentally; measure or try one's strength with, fight, vie with," from assimilated form of com-, here perhaps an intensive prefix (see com-), + tendere "to stretch" (from PIE root *ten- "to stretch"). From 1540s as "to assert, affirm, maintain." Related: Contended; contending.
According to Barnhart, the Gaelic is probably a loan-translation of Medieval Latin aqua vitae, which had been applied to intoxicating drinks since early 14c. (compare French eau de vie "brandy"). Other early spellings in English include usquebea (1706) and iskie bae (1580s). In Ireland and Scotland obtained from malt; in the U.S. commonly made from corn or rye. Spelling distinction between Scotch whisky and Irish and American whiskey is a 19c. innovation. Whisky sour is recorded from 1889.