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A verbal retaliation to an unnecessarily defensive outburst or physically defensive or evasive reaction. It is reference to the Marvel Comic Book character Captain America who uses a large round shield to defend himself from attacking foes.
"Shield Down, Captain America!" said Louis to Joe for ranting about how "everyone" was against his ideas "all the time."

When Toby winced and ducked at the cold beer Lyle tossed him, Ralph roared, "Shield Down, Captain America!"
by SAMMER of the GOODS April 28, 2009
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