The word "lain" refers to being in a horizontal or reclining position. Some common synonyms for "lain" include "rested", "reclined", "prostrate", "supine", or "flat". These words convey the same meaning of being in a lying position but with some variations in tone or context. "Rested" suggests a peaceful and relaxed state while "prostrate" implies a more submissive posture. "Supine" is often used in medical contexts to describe a patient's position while "flat" may refer to a more neutral or literal description of lying down. Overall, these synonyms offer flexibility in choosing the right word to fit the desired tone and context.