If you're ready to take your vocabulary lying down, you'll want to be familiar with the synonyms recumbent, prone, supine, and prostrate, all of which mean "lying down." Recumbent, which derives from the Latin prefix re- and the verb cumbere, meaning "to lie down," focuses on the posture or position native to sleeping or resting. Prone describes someone who is lying facedown, as, for example, in doing push-ups. Supine flips it over, suggesting the position of someone lying inert on the back, while prostrate implies a full-scale physical collapse or submission, regardless of the exact position of the defeated body. Recumbent, dating from 17th century, is the newest of the four words; the others all entered English before the 16th century.