Past tense of flex; bent a joint or muscle to show strength or ability.
From Latin 'flectere' meaning 'to bend,' the -ed suffix marks it as past tense in English, tracing back through Middle English and Old French.
When you 'flex' your muscles, you're literally doing what the Romans did 2,000 years ago with the exact same word root—humans have always had a word for showing off their strength!
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