Rotate face-up, as one's palm

Wiktionary Word definitions in Wiktionary
vb. 1 (context transitive anatomy English) To twist the forearm so as to turn the palm of the hand backwards if the forearm is pointing up, upwards if the forearm is horizontal, or forwards if the arm is pointing down; to twist the forarm by contracting ...

WordNet Word definitions in WordNet
v. turn (the hand or forearm) so that the back is downward or backward, or turn out (the leg)

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1831, "to place the hand so that the palm is turned upward," from Latin supinatus , past participle of supinare "to bend back," related to supinus (see supine ). Related: Supinated ; supinating ; supinator .