prone (adj.)

"to render prone," specifically to rotate the hand so that its palmar surface faces in the same direction as the posterior surface of the ulna, 1819, from Late Latin pronatus, past participle of pronare "to bend forward," from pronus "prone" (see prone). Related: Pronated; pronator; pronating. As an adjective, "bent into a prone position," by 1848.

"act or result of pronating, the prone position of the fore limb in which the bones of the forearm are more or less crossed and the palm of the hand is turned downward," 1660s, from French pronation, from Medieval Latin pronationem (nominative pronatio), noun of action from past-participle stem of Late Latin pronare "to bend forward," from pronus "prone" (see prone).