ColeManND87's definitions
Anytime you (or someone else) is trying to look casual but are secretly flexing in a picture or pose to make your muscles (usually arms) look bigger. This includes but not limited to, a hidden clinched fist to make your biceps look bigger, and also having your arms pressed against your body to make them look bigger.
Look at that guy, who is he trying to kid. His arms are not that big. He's just doing THE SILENT FLEX.
by ColeManND87 January 16, 2015
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