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c.1300 (late 13c. as a surname), "one who has charge of some person or thing, warden," agent noun from keep (v.). Sense of "one who carries on some business" is from mid-15c. Sporting sense (originally cricket) is from 1744. Meaning "something (or someone) worth keeping" is attested by 1999. Brother's keeper is from Genesis iv:9.
Someone or something worth keeping or trying to keep: This husband's a keeper