It literally means "butt". Commonly used in medicine, the word decubitus is used to mean "lying down". It is derived from the Latin verb “decumbere” meaning "to lie down".
When medical professionals use this term to describe the position of a patient, they first state the part of the body on which the patient is resting followed by the word "decubitus". For example, the right lateral decubitus would mean that the patient is lying on his or her right side. Another example is angina decubitus, (chest pain while lying down), [1]
In radiology, this term implies that the patient is lying down with the X-ray being taken parallel to the horizon. [2]