Present participle of beef; to complain or grumble about something, or to strengthen/build up (as in 'beef up').
Beef as a verb meaning 'to complain' emerged in 19th-century American slang, possibly derived from the muscular strength of beef cattle, suggesting 'substantial complaint.' The 'beef up' meaning (to strengthen) followed from the idea of adding substance.
It's fascinating that 'beef' is both a noun (the meat) and a verb (to complain), but the verb probably came from the noun—the idea that a serious complaint has 'weight' or 'substance' like actual beef, showing how slang builds by extending the meanings of concrete things to abstract situations.
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