The phrase "be supine" refers to laying in a horizontal position with the face up. Antonyms for this phrase include being upright, being standing, being erect, being vertical, being inclined, and being slanting. These words describe positions where the body is not lying flat on the back. For example, when a person is standing or sitting, they are in an upright position. When a person is inclined, their body is at an angle from horizontal, and when a person is slanting, their body is at an angle from vertical. Using these antonyms can help describe the position of a person or object in a more specific and descriptive way.