"Soundly" is an adverb that means deeply, profoundly, or completely. It is often used to describe the way in which a person sleeps when they are completely relaxed and at ease. There are many synonyms for the word "soundly" that can be used to express the same idea. For example, the word "serenely" means peacefully, calmly, and without disturbance. Similarly, "tranquilly" suggests a sense of calm and quietness, whereas "securely" implies a feeling of safety and protection. Other synonyms for "soundly" include "restfully", "uninterruptedly", and "thoroughly", all of which convey the idea of a deep and restful sleep.