I think it comes from Heraldry where the bird is shown with legs and wings stretched out
Nov 12 2003, 6:31 AM
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McGruff 23 year member
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Websters Online says:
Main Entry: spread-eagle
Etymology: from the spread eagle on the Great Seal of the U.S.
: marked by bombast and boastful exaggeration especially of the greatness of the U.S.