su·pine / ˈsoōˌpīn/ • adj. 1. (of a person) lying face upward. ∎  technical having the front or ventral part upward. ∎  (of the hand) with the palm upward.2. failing to act or protest as a result of moral weakness or indolence: supine in the face of racial injustice.• n. a Latin verbal noun used only in the accusative and ablative cases, esp. to denote purpose (e.g., dictu in mirabile dictu “wonderful to relate”).DERIVATIVES: su·pine·ly adv.su·pine·ness n.

supine adj. lying on one's back XV; mentally or morally inert XVII. — L. supīnus, f. *sup-, repr. in L. super above, superus higher, SUPERIOR; see -INE1.
So supine sb. form of verbal noun ending in -tum, -tū, -sum, -sū. XVI. — n. sg. of supīnus used sb.; this usage is unexpl.

supine (soo-pyn) adj.
1. lying on the back or with the face upwards.

2. (of the forearm) in the position in which the palm of the hand faces upwards. Compare prone.