Leadenly is an adjective that refers to something that's dull, heavy, and sluggish. Some other synonyms for leadenly include sluggishly, lethargically, ponderously, laboriously, slowly, and heavily. When something is leadenly, it moves as if carrying a weight or burden. This can refer to a person's movements, such as walking or lifting heavy objects, or to the pace of an event or situation. Other words that can be used in a similar context include plodding, lumbering, and cumbersome. While the word leadenly itself may not be very common, its synonyms are useful for describing anything that moves slowly and heavily.