What's the "een" in Halloween?
Old English sur "sour, tart, acid, fermented," from Proto-Germanic *sura- "sour" (cf. Old Norse surr, Middle Dutch suur, Dutch zuur, Old High German sur, German Sauer), from PIE root *suro- "sour, salty, bitter" (cf. Old Church Slavonic syru, Russian syroi "moist, raw;" Lithuanian suras "salty," suris "cheese").
French sur "sour, tart" (12c.) is a Germanic loan-word. 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.
c.1300, from sour (adj.). Cf. Old High German suren, German säuern. Related: Soured; souring.