sour (adj.)
Old English sur "sour, tart, acid, fermented," from Proto-Germanic *sura- "sour" (cognates: Old Norse surr, Middle Dutch suur, Dutch zuur, Old High German sur, German Sauer), from PIE root *suro- "sour, salty, bitter" (cognates: Old Church Slavonic syru, Russian syroi "moist, raw;" Lithuanian suras "salty," suris "cheese").

Meaning "having a peevish disposition" is from early 13c. Sense in whisky sour (1885) is "with lemon added" (1862). Sour cream is attested from 1855. French sur "sour, tart" (12c.) is a Germanic loan-word.
sour (v.)
c.1300, from sour (adj.). Compare Old High German suren, German säuern. Related: Soured; souring.