fosteryoudaoicibaDictYouDict[foster 词源字典]
foster: [OE] The etymological notion underlying foster is of ‘giving food’. Indeed, the Old English verb fōstrian meant ‘feed, nourish’, and it was not until the 13th century that secondary metaphorical senses, such as ‘rear a child’ and ‘encourage, cultivate’, began to emerge. It was a derivative of the Old English noun fōstor ‘food’, which in turn was formed from the same Germanic source, *fōth-, as produced English food.
=> food[foster etymology, foster origin, 英语词源]
foster (v.)youdaoicibaDictYouDict
Old English *fostrian "to supply with food, nourish, support," from fostor "food, nourishment, bringing up," from Proto-Germanic *fostra-, from extended form of PIE root *pa- "to protect; feed" (see food).

Meaning "to bring up a child with parental care" is from c. 1200; that of "to encourage or help grow" is early 13c. of things; 1560s of feelings, ideas, etc. Old English also had the word as an adjective meaning "in the same family but not related," in fostorfæder, fostorcild, fostormodoretc. Related: Fostered; fostering.