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A "bro" term for when you tense your muscles up tightly so that you can stand better ground and/or brace yourself before getting hit by something.

Similar to "tensing up".

Muscles usually flexed include:
- pecs
- shoulders
- biceps
"bro I almost got bodied by Chaz but i got the flex shield up in time."
"...sounds like a close call."
by Given Dodis November 29, 2007
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