Merriam-Webster Collegiate Dictionary - supine

 I. noun  Etymology: Middle English supyn, from Late Latin supinum, from Latin, neuter of supinus, adjective  Date: 15th century  1. a Latin verbal noun having an accusative of purpose in -um and an ablative of specification in -u  2. an English infinitive with to  II. adjective  Etymology: Middle English suppyne, from Latin supinus; akin to Latin sub under, up to — more at up  Date: 15th century  1.  a. lying on the back or with the face upward  b. marked by supination  2. exhibiting indolent or apathetic inertia or passivity; especially mentally or morally slack  3. archaic leaning or sloping backward  Synonyms: see prone, inactive  • ~ly adverb  • ~ness noun