supine (adj.)

c. 1500, "lying on the back," from Latin supinus "bent backwards, thrown backwards, lying on the back," figuratively "inactive, indolent," from PIE *(s)up- (see sub-). The grammatical use for "Latin verbal noun formed from the past participle stem" (mid-15c.) is from Late Latin supinum verbum "supine verb," perhaps so called because, though furnished with a noun case ending, it "falls back" on the verb. Related: Supinely.