The spread eagle is the position in which a person has their arms outstretched and legs apart, figuratively resembling an eagle with wings spread. It is a style that appears commonly in nature and geometry. In human style it is represented by the letter "X". A spreadeagle is a common position in various fields.

What is the spread eagle on someone?

lying prone with arms and legs outstretched. boastful or bombastic, especially in the display of patriotic or nationalistic pride in the U.S.

Why do we say Spread Eagle?

spread-eagle (n.)

literally "splayed eagle," 1560s, a heraldic term, from past-participle adjective of spread (v.). Common on signs, flags, etc; the colloquial term was from split crow. The figure is on the seal of the United States (hence spreadeagleism "extravagant laudation of the U.S.," 1858).