Jampack refers to something that is tightly packed or filled with a large number of things or people. Antonyms for this word may include "spacious," "empty," "sparse," "unfilled," or "unstuffed." Spacious refers to something that has ample room or a lot of space to move around. Empty means that something is devoid or lacking in substance, content, or occupancy. Sparse refers to something that is thin, scattered, or thinly spread out. Unfilled indicates that something is vacant or unoccupied. Lastly, unstuffed refers to something that is not burdened or crowded with people or things. These antonyms could be used to describe an environment, a physical space or a situation depending on the context.