"Fell knees" can be described using various synonyms to express the same meaning. Some synonyms that can be used for "fell knees" are collapsed knees, crumpled knees, weakened knees, dropped knees, and buckled knees. All of these words suggest that someone's knees have suddenly given way, causing a loss of balance or stability. The use of synonyms adds depth to the description of a person's physical state, bringing them to life in the imagination of the reader. Choosing the right synonym can help create a more vivid and impactful image of a scene, which can be an essential tool for storytelling and communication.