When a woman (or possibly a man) wears a button-down shirt that is a little too tight, resulting in the fabric straining at the buttons, and forming a gap. This allows friends, family, and/or the general public to have a glimpse of your side-boob.
"Oh man! That girl should really put a sweater on."

"Why?"

"Because she's totally gap-flashin' the entire coffee shop!"
by america's cynic January 27, 2010
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