The word "most slumbery" is a unique phrase that refers to something that is sleepy or drowsy to a great extent. Trying to find synonyms for this phrase, we can use terms like "extremely sleepy," "super drowsy," "profoundly somnolent," "heavily lethargic," "tired to the extent of exhaustion," or "deeply unconscious." Other options for synonyms include "intensely dormant," "overwhelmingly torpid," "extremely fatigued," "completely drained," and "exhausted beyond measure." These synonyms can help us describe the feeling of being intensely tired or sleepy in a way that makes our language more vivid and evocative.