"add strength," 1941, from college slang, from beef (n.) in slang sense of "muscle-power" (1851).
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beef n.
c. 1300, "an ox, bull, or cow," also the flesh of one when killed, used as food, from Old French buef "ox; beef; ox hide" (11c., Modern French boeuf), from Latin bovem (nominative bos, genitive bovis) "ox, cow," from PIE root *gwou- "ox, bull, cow." The original plural in the animal sense was beeves.